Without a doubt, Sir Alex Ferguson is the most successful manager in recent footballing history. In a 24 year reign at the helm of United, Ferguson has acquired 11 Premier League crowns, 2 European Cups, and numerous League and FA Cups.

However, his success hasn't just come about through his managerial prowess, but also through his activity in the transfer windows. In the following slides I have listed some of the greatest signings that Sir Alex has ever made as United manager.

20. Carlos Tevez

He was at a good club one point in time....

19. Javier "Chicharito" Hernandez

From: Guadalajara

Cost: Undisclosed

Year: 2010

Chicharito was little known outside of Mexican football, however, scouting teams at United were able to see the potential that he possessed. Prior to the 2010 World Cup, Manchester United made the deal to sign Hernandez from Guadalajara.

Hernandez showed his quality in the tournament, by scoring a fantastic goal against tournament favorites, Argentina. Since his move to Manchester, Hernandez has mainly come off the bench, but in doing so he has scored many important goals.

In the few games he has played this season, Hernandez has scored 11 goals and seems to not be slowing down. Chicharito looks to be one of the best signings that Sir Alex has ever made with a striker.

18. Dimitar Berbatov

From: Tottenham Hotspur

Cost: £30 million

Year: 2008

Called lazy by many, Berbatov's playing style angered many United fans. However, his class is undeniable.

His effortless control of the football is mesmerizing, and he has just started to break into top form this season. Top scorer in the Premier League, Berbatov has led the United charge in a season which Wayne Rooney has not been in top form.

To many, £30 million may have been a lot to pay for him, but he is now starting to show his true class for United.

17. Jaap Stam

From: PSV Eindhoven

Cost: £10.6 million

Year: 1998

Jaap Stam was one of the best defenders to play in the Premier League, let alone a United top.

He only had a short 3 year stint at United, however, in that time he won the Premier League 3 times and also had a major part in United's run in for the 1999 European Cup.

Unfortunately for United, Stam was sold to Lazio as a tensions began to mount between himself and the manager.

16. Patrice Evra

From: Monaco

Cost: £5.5 million

Year: 2006

Evra joined United after much speculation that he may be heading to Inter Milan. However, United put pen to paper and signed him in 2006.

Since then, Evra has grown to be not only one of the best left backs in the world, but also one of the best defenders. His companionship with Vidic and Ferdinand make United's back line one of the most watertight in all of world football.

15. Dwight Yorke

From: Aston Villa

Cost: £12.6 million

Year: 1998

Yorke joined United at just the right time. Upon his arrival, United won the European Cup, Premier League and FA Cup to complete a historic treble. His strike partnership with Andy Cole was one of the most productive in all of the Premier League.

In 95 appearances, Yorke scored 47 goals.

14. Denis Irwin

From: Oldham Athletic

Cost: £625,000

Year: 1990

With nearly 400 appearances for United, Irwin was a steady inclusion in the back four.

Irwin won 7 Premier League titles, 3 FA Cups and a European Cup in 1999. Sir Alex made a great buy when he went for Irwin, and his performances over his 12 United years showed once again the genius behind Ferguson's purchases. 

13. Nani

From: Sporting CP

Cost: Undisclosed

Year: 2007

Following in the footsteps of Cristiano Ronaldo, Nani left Sporting to join Manchester United.

While Ronaldo was still at the club, Nani never produced the form that was expected from him. There was speculation of him leaving the club, but that changed with the selling of Cristiano Ronaldo.

With Ronaldo's departure, Nani has stepped up to fill in Ronaldo's shoes. Though Ronaldo himself may be irreplaceable, Nani has provided United with the attacking spark that Ronaldo had.

12. Rio Ferdinand

From: Leeds United

Cost: £29 million

Year: 2002

At £29 million, Manchester United's purchase of Rio made him the most expensive British footballer in history.

He has proved his worth over the years by being a consistent defender and leader in the back. His partnership with Nemanja Vidic has been called by many as the best defensive pairing in the world. To this day they still foil attackers match after match.

Though he has been set back by injuries over the past few years, when in form Rio Ferdinand is one of the best defenders in the world.

11. Edwin Van Der Sar

From: Fulham

Cost: Reported £2 million (undisclosed)

Year: 2005

A good few years between the sticks at Fulham eventually caught the eye of Sir Alex. It was about time too, as United had been struggling to fill in the gaping hole in net after Peter Schmeichel's departure.

Van der Sar had an impressive start at United, winning a the league in only a few years of being there.

Edwin has been making clinical saves ever since his move to Old Trafford. None could possibly be more meaningful than the penalty save against Nicola Anelka in the 2008 Champions League final, a save that won the Cup for United.

Upon his retirement at the end of this season, United are going to have to look hard for another keeper of his standard.

10. Andy Cole

From: Newcastle United

Cost: £7 million

Year: 1995

Purchased from Newcastle United, a then title contender, Andy Cole made the shocking switch to Manchester United.

Cole turned out to be one of the most dangerous strikers in the Premier League, scoring just under 100 goals in 195 games. He was part of the team that won the 1999 European Cup final, and will go down as one of United's best strikers in history.

9. Ruud Van Nistelrooy

From: PSV Eindhoven

Cost: £19 million

Year: 2001

Van Nistelrooy had many successful years at United, winning the Premier League, FA cup and League Cup.

He became one of Manchester United's most dangerous front men in his spell with the club, scoring 95 goals in just 150 matches. Ferguson found a gem in Ruud and wasted no time implementing him into his plans.

Unfortunately, a fall out with Sir Alex marked the end of his United career.

8. Ole Gunnar Solskjaer

From: Molde

Cost: £1.5 million

Year: 1996

The Baby Faced Assassin.

Solskjaer became an instant United hero with that goal in Barcelona. When all hope seemed lost in the 1999 Champions League Final, Teddy Sheringham equalized in added time. Then along came Ole to prod home the winner in the dying embers of the match to hand United a second European triumph and a historic Treble winning season.

Solskjaer has always been a fan favorite; another brilliant spot by Ferguson to sign him up.

7. Nemanja Vidic

From: Spartak Moscow

Cost: £7 million

Year: 2006

After much speculation and probing, Manchester United finally got pen to paper for Vidic's signature in 2006.

Almost instantly, Vidic formed a dominating partnership with Rio Ferdinand, which has lasted to this day. Since his move to United, Vidic has constantly improved and is arguably the best defender in the world right now. He has gone on to become United captain, as they look for their 12th Premier League title.

6. Steve Bruce

From: Norwich city

Cost: £825,000

Year: 1987

Steve Bruce was purchased by Manchester United in order to bolster their defense and finish higher in the then First Division.

It took a few years for United to gain dominance in English football, however, the 92-93 season was a breakout one for them. United won the Premier League title, and 2 late goals from Bruce against Sheffield Wednesday proved to be decisive in their gaining of the title.

5. Wayne Rooney

From: Everton

Cost: £27 million

Year: 2004

In the wake of Euro 2004 a plethora of teams were trying to get Wayne's signature. In the end it was Fergie who managed to lure the 18 year old away from Goodison Park.

Rooney's temperament may be questionable at times (along with his actions off the field), yet his ability is unquestionable. His goal scoring record speaks for itself, and even in seasons such as the 2010-2011 season when his goal scoring is sub par, he more than makes up for it with his work rate and assists in play.

4. Peter Schmeichel

From: Brondby

Cost: £530,000

Year: 1991

The greatest goalkeeper to ever wear the number 1 for Manchester United.

Schmeichel was a domineering force between the sticks, winning a number of titles and self awards with his time at United. The perfect send off for Schmeichel was winning the 1999 Champions League, where he could be spotted doing his famous cartwheel.

Until Edwin Van der Sar's arrival, United was lost in the goalkeeper position after his departure.

3. Roy Keane

From: Nottingham Forest

Cost: £3.75 million

Year: 1994

Captain Fantastic.

The tenacity of the long serving United captain was unmatched in the Premier League. For what Keane lacked in pace and skill, he more than made up for in his work ethic and leadership.

A centerpiece in United's midfield for much of the 90's and early millennium, Roy helped United win title after title. 

And who could ever forget those epic clashes between him an Patrick Vieira?

2. Eric Cantona

From: Leeds United

Cost: £1.2 million

Year: 1992

After only one season at Leeds United, Cantona made the switch over to Manchester under the watchful eye for Sir Alex.

Cantona had a prolific goal scoring record, with nearly a goal every two matches. He was so beloved by the United faithful that he was dubbed King Cantona by some and god by others. His legend still rings round the stands of Old Trafford in the form of Ooh Aah Cantona and forever immortalized at Christmas time in the form of 12 Days of Cantona.

Long live the King.

1. Cristiano Ronaldo

From: Sporting CP

Cost: £12.24 million

Year: 2003

A global phenomenon, Ronaldo made a name for himself in the #7 United shirt. His knack of scoring goals, great footwork, and natural ability helped him lead United to numerous titles including the European Cup.

The way he built himself up from a player from a small club, to a £80 million superstar, speaks volumes for the amount of work he put in at Manchester United.

With 196 games for United and 84 goals, Ronaldo is perhaps Sir Alex Ferguson's best signing.

Conclusion

Sir Alex Ferguson's signings are part of what made this club as great as it is now.

The ranking of the players in this list is of course not completely accurate, as everyone has their own opinions. If you feel some are out of position feel free to give your ideas.