The football transfer window is heating up, and there are sure to be some deals this summer that reshape the clubs ad leagues they involve.

Most of the big transfer sagas this summer will go on until the transfer deadline day, so for now, let's take a look back at some previous transfers.

Many of the greatest clubs in football history have had at least one transfer move that has defined the club's path to greatness, and some transfers have been so big, they've changed the game.

So which one comes in as the most important of all time?

Chime in with who you think was left out or who is out of place. 

25. David Beckham to LA Galaxy/Pele to New York Cosmos

David Beckham is one of the biggest stars world football has ever seen, and his mammoth transfer to MLS side LA Galaxy showed real determination by the American league.

MLS knew they needed some star power to help get the league some much-needed exposure. 

The deal was originally reported to be worth about $250 million total including wages, revenue and endorsements.

But Beckham's deal would never have happened had Pele not been signed by the New York Cosmos in 1975.

Pele played two years with the Cosmos, and though he was past his prime, he still greatly increased the play of the team and brought a lot of attention to the North American Soccer League.

Beckham has greatly increased the exposure of the MLS, and he's brought other world stars to the league as well. It's also led to many of Europe's top teams using the States as a summer training base and scheduling high-profile friendlies against MLS clubs.

But Pele is Pele.

You decide which of these transfers was more important.  

24. Luis Figo to Real Madrid

Not many footballers play for both Barcelona and Real Madrid, and even fewer go from one straight to the other.

So Luis Figo's move from Barca to Real in the first Galactico era was met with plenty of animosity from the Catalans. 

He won two La Liga titles with each club, but he added the Champions League trophy with Madrid. 

23. Robinho to Manchester City

Robinho was Manchester City's first big signing under owner Sheikh Mansour. He was signed on deadline day in 2008 from Real Madrid, the same day Abu Diabi United Group's ownership of the club was finalized.

Robinho was bought for £32.5 million, and though he did not really pan out as a player for City, his was the signing that started City's surge in spending to become one of Europe's top clubs.  

22. Steve McManaman to Real Madrid

Steve McManaman has had some of the most drawn-out contract/transfer sagas of any footballer. 

After two years of negotiations with Liverpool broke down, he decided to exercise his Bosman right and leave the club as a free agent in 1999, called the most important Bosman ruling since Jean-Marc Bosman himself.

Initially, there was a deal to send Macca to Barcelona, but that fell through.

He eventually went to Real Madrid where he became highest paid Englishman for a time and the first British footballer to win the Champions League twice.  

21. David Beckham to Real Madrid

David Beckham's relationship with Sir Alex Ferguson was deteriorating in the 2002-03 season, and transfer speculation was rife. 

United originally wanted to sell the former golden boy to Barcelona, but Beckham preferred to join the Galacticos of Real Madrid.

The move to Real helped to increase Beckham's global popularity, but Manchester United has had more success of the pitch.  

20. Wayne Rooney to Manchester United

Wayne Rooney burst onto the scene with Everton as a teenager and at 17, was the youngest player to ever earn a spot on the England senior side when he debuted.

He made a mammoth move to Manchester United in 2004 when he was 18 worth £25.6 million, the most ever for any player under 20. 

He is still hated by Everton fans for the way he left the club, but the seven trophies he's won with United probably make up for that. 

19. Samuel Eto'o/Zlatan Ibrahimovic Swap Deal

The big transfer of 2009 that didn't involve Real Madrid was the swap deal between Inter Milan and Barcelona with Zlatan Ibrahimovic and Samuel Eto'o. 

Barca got Ibra and his sterling Champions League record (note the sarcasm please), and Inter got Eto'o and €46 million.  

Ibra won La Liga with Barcelona in 2009-10 while Eto'o won the Serie A, the Coppa Italia and the Champions League, becoming the only footballer to win the treble in two straight seasons.

Suave piece of business by Inter. 

18. Didier Drogba to Chelsea

Didier Drogba wasn't the first of Roman Abramovich's mega-signings for Chelsea, but he's been arguably his best. 

Drogba came from Marseille in 2004 for £24 million (a bargain price if you think about it now), and he's gone on to score 144 goals in 306 appearances and won three Premier League titles, two League Cups and four FA Cups. 

17. Ronaldo to Real Madrid

Not only has Ronaldo played for both Barcelona and Real Madrid, but he's also played for both AC and Inter Milan, among others. 

And he won two World Cups with Brazil.

He moved to Real from Inter as one of the most-anticipated Galacticos in 2002 for €39 million and immediately broke jersey sales records. He scored 104 total goals in 177 appearances over five years for Madrid, his longest stint at any of his seven clubs. 

16. Cesc Fabregas to Arsenal

Cesc Fabregas decided to leave Barcelona at the age of 16 because he didn't think he'd get a lot of opportunities.

He was right about that, but he probably didn't know that his move would lead to a transfer saga that lasts all summer long, year after year and will keep going until he finally just goes back to Barca. 

Besides that, just imagine what Barca would be like with Fabregas. Would he be starting in favor of Andres Iniesta? Or would Cesc have left just several years later because he couldn't get into the starting lineup?

And where would Arsenal be?

15. Dixie Dean to Everton

William "Dixie"" Dean is the most prolific scorer in English football history, and he cost Everton £3,000 from Tranmere Rovers. 

Even in 1925, that's a bargain.

He scored 32 goals in his first full season, famously scored from a header just days after he fractured his skull in a motorcycle accident and is still the only player in English football to score 60 league goals in one season (1927-28). 

He was the first player to wear the No. 9 and finished his career at Everton with 395 goals in 447 appearances (349 in 399 league). Overall, he had 425 goals in 489 appearances in English football. 

Photo courtesy of wikipedia.org. 

14. Ruud Van Nistelrooy to Manchester United

Ruud van Nistelrooy came to Manchester United in 2001 for a the- record fee of £19 million.

In his first season, he became the highest scoring foreign, first-year Premier League player with 23 league goals in 32 games (he scored 36 overall), and he scored in eight consecutive league games.

He picked up right where he left off the next season and finished his United career in 2006 with 150 goals in 219 appearances, is still the club's leading scorer in Europe and won the Premier League, FA Cup and League Cup each once.

In 2007-08, Fernando Torres broke van Nistelrooy's Premier League scoring record by notching 24 league goals in his first season with Liverpool, though he scored only 33 overall.

13. Lionel Messi to Barcelona

Lionel Messi hasn't always been with Barcelona, and just imagine where they'd be without him.

Messi came to Barcelona from Argentina's Newell's Old Boys when he was 13 because they could afford the medical bills ($900 a month) to treat his growth hormone deficiency. 

He made his league debut for Barca when he was 17, and so far, the barely 24-year-old has five La Liga titles, one Copa del Rey and three Champions League trophies. 

12. Roy Keane to Manchester United

Roy Keane was supposed to be signing for Blackburn and Kenny Dalglish in 1993 after a £4 million fee was agreed, but Alex Ferguson hijacked the Irishman and brought him to Manchester United for a then British transfer record £3.75 million.

Keane came to a United team that had just won its first league title since 1967, but he quickly established himself in the first team.

His fiery temper led to 11 total red cards in his career at United, and he was handed the captain's armband when Eric Cantona retired in 1997. 

In 12 seasons with United, Keane won seven Premier League titles, four FA Cups and one Champions League trophy.  

11. Alan Shearer to Blackburn, Newcastle

In 1992, Kenny Dalglish nabbed English up-and-comer Alan Shearer for Blackburn at a then British transfer record £3.3 million (which was obviously broken the next year with Keane).

Shearer, the Premier League's top goal scorer, helped Blackburn to win the Premier League in the 1994-95 season. He scored 34 league goals that season—equaling Andrew Cole's record set the year before—a record that still stands today.  

Blackburn was the club that put Shearer on the map, and Newcastle paid a then world record £15 million for Shearer in 1996, four years and 130 goals in 171 appearances later.

At Newcastle, he further established himself as one of the greatest English strikers of all time, scoring 206 goals in 404 appearances over 10 seasons.  

10. Diego Maradona to Napoli

Diego Maradona had already captured the world's attention with Argentina by the time he signed for Barcelona, but it wasn't until he moved to Napoli that his game really hit its peak.  

He signed for Barcelona from Boca Juniors for a record £5 million in 1982, but he was frequently involved in bust ups with his manager and did not last long at Barca.

He moved to Italy in 1984 for another record fee £6.9 million and led Napoli on its most dominant stretch in the Serie A.

With Maradon, Napoli won its first Serie A title in 1987 and the Coppa Italia the same year. They won the Serie A again in 1990.

His time in Italy was also marred by drug and sex scandals, and he frequently missed practices. 

9. Gianfranco Zola to Chelsea

Gianfranco Zola was a teammate of Maradona's at Napoli, and he went on to become arguably Chelsea's greatest ever player.

Zola arrived in London in November of 1996 for £4.5 million and at the end of the season, was named the FWA Player of the Year (first Chelsea player), the only player to win the award having not played the full league season at his club.

He was a key member in Chelsea's first ever Champions League campaign, scoring three goals on the Blues' run to the quarterfinals. 

He spent seven seasons at Chelsea and won two FA Cups and one League Cup with Chelsea. Though his No. 25 has not officially been retired, it has not been worn by another Chelsea player since. 

8. Thierry Henry to Arsenal

Arsene Wenger had managed Thierry Henry at Monaco, and he later brought him to Arsenal in what is probably the Frenchman's best ever signing. 

Henry moved to London in 1999 for £11 million, and he became a world-class footballer with the Gunners. Though he failed to score in his first eight games, he finished his first season with 26 goals, 17 in the league as Arsenal finished in second place.

He had 226 goals and 92 assists in 369 appearances for Arsenal. 

He led Arsenal to the Double (Premier League and FA Cup) in 2001-02, and in 2003 and 2004, he was the PFA and FWA Player of the Year and runner-up in the FIFA World Player of the Year.

He was a member of Arsenal's legendary "Invincibles" and moved to Barcelona in 2007 after two Premier League titles and three FA Cups. 

7. Dutch Contingent to AC Milan

Frank Rijkaard joined his Dutch teammates Marco van Basten and Ruud Gullit at AC Milan in 1988, and the three of them transformed the club.

AC Milan won back-to-back European Cups in 1989 and 1990, the first two for the club, and three Serie A titles.

Rijkaard went from a center-back to a world-class midfielder and scored the only goal in the 1990 European Cup Final. 

Van Basten had joined the year prior when Milan's won its first Scudetto in eight years though he was troubled by injury all season long.

Gullit also joined in 1987, and he scored two of Milan's four goals in their first European Cup final victory over Steaua Bucharest in 1989. 

Rijkaard and Gullit both left the club in 1993, and van Basten retired two years later after they had turned the club into one of Europe's elite.   

6. Cristiano Ronaldo to Manchester United, Real Madrid

Cristiano Ronaldo came to Manchester United in 2003 for the absolute bargain price of €15 million.

Ronaldo said he didn't want the pressure of the No. 7 shirt when he first arrived, but Sir Alex Ferguson said the pressure would motivate him. 

He did not really become a star until 2006-07. He broke the 20-goal mark and won his first Premier League title with United though he had already won the FA Cup.

He finished second to Kaka in the Ballon d'Or voting in 2007. 

The next year he scored 42 goals in 49 appearances as United won their third straight league title and the Champions League. He was also the European Golden Shoe and Ballon d'Or winner for 2008 and the first Premier League player to be named FIFA World Player of the Year.

In 2009, Real Madrid paid a world record £80 million for Ronaldo's services as their marquee signing for the second Galacticos era.

He scored 33 goals in his first season, though Madrid could not win a trophy. 

In 2010-11, he scored 53 total goals in 54 appearances and became the only footballer to win the European Golden Shoe in two different leagues. His 40 league goals also set a new record for most in a La Liga season.

5. Johan Cruyff to Barcelona

Johan Cruyff won eight Dutch league titles and three European Cups with Ajax before moving to Barcelona in 1973 for $2 million. 

He immediately won over the Catalan fans by beating Real Madrid 5-0 on the way to winning their first league title since 1960. 

He scored possibly his most famous goal for Barcelona, the "Phantom Goal." With the ball wide of the far post and at neck-height and his body already turning away from goal, Cruyff lept into the air and poked the ball past the keeper.

He played five seasons with Barca, wining one league title and one Copa del Rey. He returned to manage the club in 1988 and led the club to four La Liga titles, one Copa del Rey and the club's first European Cup in 1992.  

4. Kenny Dalglish to Liverpool

Kenny Dalglish was already a star for Celtic when he came to Liverpool in 1977, as evidenced by his British record transfer fee of £440,000.

He took Kevin Keegan's No. 7 shirt and made it his own almost immediately. He scored 31 goals in his first season, including the winning goal in the 1978 European Cup final. 

He scored 21 league goals in the next season and didn't even miss a league match until his fourth season with Liverpool. 

He was an ever-present in his playing days and then took over as player-manager in 1985. That season, he led Liverpool to its first ever Double (league title and FA Cup). He won the league nine times, the FA Cup three times, the League Cup four times and the European Cup three times.  

He abruptly retired in 1991, but his return in January of this year has reinvigorated the club as he looks to take them back to where they were when he left. 

3. Eric Cantona to Manchester United

Eric Cantona was a key player for Leeds United before he moved to their old rivals Manchester United for £1.2 million in November of 1992. 

Goal scoring had been a problem for the Red Devils in the inaugural Premier League campaign, but Cantona's goal scoring prowess and his ability to create chances for his teammates quickly fixed that.

He helped United win the title that season, becoming the first player to win back-to-back titles in the English top flight with two different teams.

Cantona went on to claim four league titles in five years with United in addition to two FA Cups, and he was always entertaining, due in large part to the fact that controversy was never far away.  

2. Zinedine Zidane to Juventus, Real Madrid

Juventus purchased Zidane for £3.2 million in 1996, without much fight from Blackburn owner Jack Walker, and he won two Serie A titles in his five seasons with the club. 

In 2001, Zinedine Zidane moved to Real Madrid from Juventus for a then world record €75 million. 

Zidane was the most marquee signing of the first Galacticos era, and in his five years with Madrid, he won the Champions League, La Liga and Copa del Rey each once. 

1. Alfredo Di Stefano to Real Madrid

Everything has a reason, and the reason that El Clasico isn't just another match is Alfredo Di Stefano.

His transfer was one of the most fought over in football history, and it's definitely the most important. 

Di Stefano was originally thought to be headed to Barcelona from Millionarios in 1953, having signed a deal with both the club and FIFA. However, Millionarios had apparently not signed off on the deal and would not allow the player to leave.

FIFA ordered the Spanish Federation to sort everything out, and Real Madrid president Santaigo Bernabeu took advantage of the confusion and discontent in the Barcelona camp.

In the end, the federation ruled that Di Stefano would play four seasons in Spain, two for each Real and Barca. Barca fans and management were not happy at all and eventually dropped their deal with Di Stefano.

The Spanish Federation also banned foreign players for a time as a result of this saga.

Di Stefano played 11 seasons with Madrid, scoring 307 goals in 396 appearances and winning eight La Liga titles, one Copa del Rey and five European Cups. 

Photo courtesy of wikipedia.org