In the past few years, some of MMA's greatest fighters have retired from the sport. Although the likes of Chuck Liddell, Randy Couture and Mirko Filipovic were part of what was only the first significant generation of MMA fighters, they left behind a legacy that will last forever in the sport's history books.

After his upcoming bout against Forest Griffin, the UFC's longest-reigning light heavyweight champion, Tito Ortiz, will join the ranks of the retired legends. Considering his age, current middleweight champion Anderson Silva may not be far behind.

While the aforementioned fighters, along with the likes of Fedor Emelianenko and Georges St-Pierre, have already cemented their place in the MMA's history, current champions like Jon Jones, Jose Aldo and Dominick Cruz are all well on their way toward becoming legends despite their young ages.

As we prepare to say goodbye to another one of MMA's greatest fighters in Ortiz and bare witness to the UFC current batch of champions, some of the most talented fighters to ever compete in the sport, let's take a look at the five greatest fighters in every weight division.

Since it has a very short history, even outside of the UFC, the flyweight division will not be included in the following rankings.

Heavyweight

1. Fedor Emelianenko

Emelianenko's recent three-fight losing streak may have hurt him in the present, but it shouldn't impact his legacy after going undefeated for practically a decade, with his only loss during that time coming via cut due to an accidental elbow. 

Over the course of his career, Emelianenko defended the Pride heavyweight title twice, won the 2004 Pride Heavyweight Grand Prix and defeated the likes of Antonio Rodrigo Nogueira, Mirko Filipovic, Mark Coleman, Tim Sylvia and Andrei Arlovski.

2. Antonio Rodrigo Nogueira

Though he has had some setbacks in the UFC, Nogueira is the only heavyweight to wear both Pride and UFC belts during his career. Had Emelianenko not been around during his Pride tenure, Nogueira may have gone down as the greatest heavyweight ever.

Coleman, Filipovic, Sylvia, Josh Barnett and Randy Couture all came up short against Nogueira's excellent combination of boxing and jiu-jitsu throughout the Brazilian's 13 years and counting in MMA.

3. Randy Couture

Had Couture focused his entire career on one division, he very easily could have become the greatest heavyweight or light heavyweight in MMA history. Either way, "The Natural" is still one of the greatest fighters of all-time.

During his illustrious career, Couture won and defended the heavyweight title a total of three times each. Along the way, Couture defeated heavyweight greats Maurice Smith, Pedro Rizzo, Sylvia and Coleman. 

4. Frank Mir

A two-time heavyweight champion, Mir slides into one of the two spots on this list that could belong to many different fighters. 

A rough patch after his career-threatening motorcycle accident provides fuel for an argument against including Mir on this list, but wins over Nogueira, Filipovic, Sylvia and Brock Lesnar make Mir a valid candidate for a spot among the best heavyweights ever.

5. Mirko Filipovic

Filipovic was a massive disappointment after being signed to compete in the UFC, but he should not be remembered for that lowly portion of his career.

Prior to signing with the UFC, Filipovic defeated Coleman, Barnett, Igor Vovchanchyn and many others en route to winning the 2006 Pride Openweight Grand Prix. More than just his material accolades, Filipovic's kickboxing style also helped MMA evolve.

Light Heavyweight

1. Chuck Liddell

Although he may not have the most consecutive light heavyweight title defenses in UFC history, Chuck Liddell's reign as 205-pound champion was as impressive as any.

Liddell finished his career with losses in five of his last six fights, but "The Iceman" picked up four title defenses and wins over Tito Ortiz, Randy Couture and Wanderlei Silva before calling it quits. 

2. Jon Jones

At 24 years old, Jones is already among the best light heavyweights in the history of the sport. 

The phenom has run through all competition that has been set before him and has currently defeated former UFC champions in four consecutive fights. With a win over former Pride champion Dan Henderson in his next fight, Jones could already be considered the best 205-pound fighter ever.

3. Wanderlei Silva

It seems so long ago, but Silva was once one of the most dominant fighters in the sport.

While with Pride, Silva defeated Henderson, Kazushi Sakuraba, Quinton Jackson and Ricardo Arona on his way to four defenses of the Pride belt. The Brazilian also won the 2003 Pride Grand Prix during his career with the Japanese promotion.

4. Frank Shamrock

Shamrock was the first 205-pound champion in UFC history and only relinquished his belt due to a temporary retirement after defeating Ortiz.

Had Shamrock continued defending his UFC belt, he would likely still hold the record for most consecutive title defenses in the division's history.

5. Tito Ortiz

With Shamrock vacating his title, a door was opened for "The Huntington Beach Bad Boy" to become the most dominant light heavyweight champion in MMA history. 

After losing to Shamrock, Ortiz won six straight fights to set a light heavyweight record with five consecutive title defenses. That record still stands today, though Jones appears determined to break it in the near future.

Middleweight

1. Anderson Silva

With nine straight title defenses, Silva makes choosing the greatest middleweight in MMA history a simple task. 

For more than six years, "The Spider" has gone undefeated and was only recently in danger of having his streak broken against Chael Sonnen. Aside from that lone fight in which he fought through a rib injury, Silva has appeared unbeatable at middleweight.

2. Rich Franklin

Had it not been for Silva, Franklin may have been the fighter to retain the middleweight belt for years. However, in two meetings with the Brazilian, Franklin was finished with knee both times and chased from the 185-pound division.

Still, outside of Silva, Franklin is the only UFC middleweight to defend the title more than once as champion. Now, "Ace" is making a return to the middleweight division for a late-career run at a Cinderella title shot, but he will have to overcome great odds to climb the ladder.

3. Dan Henderson

"Hendo" has had success in several different divisions throughout his career, but his resume looks most impressive in a 185-pound division that has not been as stacked with talent as the 205-pound division.

While with Pride, Henderson defeated Murilo Bustamante on two occasions and became the promotion's welterweight champion, which most closely resembles today's middleweight division.

4. Paulo Filho

For the first seven years of his career, Filho went undefeated and claimed the WEC middleweight title. On his way to the championship, the Brazilian defeated Sonnen, Kazuo Misaki, Ryo Chonan, Murilo Rua, Amar Suloev, Ikuhisa Minowa and many more.

While Filho has fallen off of the map since a strange rematch with Sonnen in which he appeared mentally unstable, it's hard to deny he was one of the best middleweights in all of MMA at one point.

5. Murilo Bustamante

In his first 11 professional fights, Bustamante's only loss came in a light heavyweight bout against Chuck Liddell. 

After losing to Liddell, Bustamante returned to the middleweight division, where he become middleweight champion and defended his title once before leaving for Pride.

Ultimately, Bustamante was not as successful in Pride, but he did reach the finals of the 2005 Pride Welterweight Grand Prix, where he suffered a split decision loss against Dan Henderson. Had Bustamante been awarded the win in that fight, his position on this list could be very different.

Welterweight

1. Georges St-Pierre

With his win over Jake Shields, St-Pierre surpassed Matt Hughes to take the UFC record for the most consecutive welterweight title defenses.

Since St-Pierre also defeated Hughes twice in the three-fight series between the two fighters, the Canadian should now be considered the indisputable greatest 170-pound fighter ever.

2. Matt Hughes

Though St-Pierre has taken his spot at the top of the welterweight division's all-time rankings, Hughes will forever be remembered as one of the best fighters in the history of the sport.

While he no longer holds the record for most consecutive title defenses, Hughes did have two separate reigns as 170-pound champion in which he picked up four or more wins before losing the title.

3. Pat Miletich

The first welterweight champion in UFC history, though it was known as the lightweight division at that time, Miletich defended his belt four straight times and was on his way to a fifth before getting caught in a bulldog choke by Carlos Newton.

4. Carlos Condit

Considering he only just won a UFC title for the first time, it may seem a little early for Condit to be included on an all-time list, but "The Natural Born Killer" has beaten some of the best welterweight in the sport and had already proven himself before entering the Octagon for the first time.

In addition to wins over Nick Diaz, Rory MacDonald and Jake Ellenberger, Condit defended the WEC welterweight strap three times before joining the UFC.

5. Nick Diaz

Diaz's UFC run came up short of a championship, but the former Strikeforce and WEC welterweight champion finally proved he was an elite 170-pound fighter by defeating B.J. Penn and nearly earning an interim belt in a narrow decision loss against Condit.

Lightweight

1. B.J. Penn

There's a valid argument that Penn has lost his spot atop the lightweight division's all-time rankings, as Frankie Edgar defeated "The Prodigy" twice and matched his record three consecutive lightweight title defenses.

However, Penn defeated three different fighters during his reign as champion and never retained the belt via draw. Also, the Hawaiian likely could have picked up a fourth title defense if he had not taken a shot at becoming the simultaneous champion of the lightweight and welterweight divisions during his time as 155-pound king.

2. Frankie Edgar

With a win over Benson Henderson in his most recent fight, Edgar could have solidified his spot as the best lightweight ever by breaking Penn's record for most consecutive title defenses. After losing to Henderson, Edgar is now tied for the record and still sits behind Penn in the all-time lightweights rankings in the minds of many. 

3. Jens Pulver

Though Pulver may have damaged his legacy by sticking around in the sport for too long, he remains one of the best lightweights to ever compete in MMA. 

With wins over the likes of Penn and Caol Uno, Pulver defended the UFC title twice before vacating it due to a contract dispute. Had Pulver remained with the UFC, he may have sat atop this list of the best lightweights in MMA history.

4. Benson Henderson

With his win over Edgar in February, Henderson has validated the strength of the former WEC lightweight division by taking the UFC title. Now, the former champion of the defunct organization will set his sights on defending the UFC title against the man he took it from in an immediate rematch.

5. Takanori Gomi

Gomi may not be having a ton of success inside the Octagon, but the Japanese fighter was once one of the best lightweight fighters in the sport.

Over the course of his career, the former Pride and Shooto champion defeated the likes of Rumina Sato, Jens Pulver, Tatsuya Kawajiri, Hayato Sakurai and Tyson Griffin.

Featherweight

1. Jose Aldo

With wins in six straight featherweight title fights, Aldo has taken over Urijah Faber's spot as the greatest featherweight in MMA history.

In 22 career fights, Aldo's only loss came in a lightweight bout in November 2005. Since then, Aldo has defeated great opponents such as Faber, Mike Brown and Kenny Florian.

2. Urijah Faber

Earlier in his career, Faber recorded five consecutive title defenses as WEC featherweight champion. While Faber is now an elite bantamweight, he was once the best 145-pound fighter in the world before running into a tough challenger in Mike Brown.

Faber has a chance to defend his spot atop this list when he faced off against Aldo in April 2010, but "The California Kid" was unable to find an answer for the Brazilian's dynamic striking ability. 

3. Mike Brown

Though he holds two victories over Faber, Brown was only able to defend the WEC featherweight title on two occasions before Aldo came along and took over the division.

In addition to his wins over Faber, Brown also defeated Mark Hominick, Jeff Curran and Leonard Garcia en route to reaching the top of the 145-pound division. 

4. Norifumi Yamamoto

Before making an unsuccessful move to the bantamweight division, Yamamoto went on an outstanding run as a 145-pound fighter in Japan.

For more than five years, Yamamoto went undefeated under the K-1 Hero's banner and recorded the fastest knockout in the promotion's history with a flying knee four seconds into his fight against Kazuyuki Miyata.

5. Hatsu Hioki

In his past 16 fights, Hioki's only loss came in a split decision against Michihiro Omigawa. During that time, Hioki became a Sengoku and Shooto champion before transitioning to the UFC, where he is now one of the top contenders for Aldo's belt.

Bantamweight

1. Dominick Cruz

Aside from a loss to Urijah Faber at featherweight, Cruz's MMA career has been spotless. With four consecutive title defenses, Cruz has surpassed Miguel Torres as the bantamweight champion with the most consecutive title defenses in Zuffa history.

2. Miguel Torres

Torres has quickly fallen out of bantamweight title contention in the UFC, but he was once one of the best pound-for-pound fighters in the world as the WEC's most dominant 135-pound champion.

During his reign as bantamweight champion, Torres picked up three straight title defenses before suffering a knockout loss to Brian Bowles.

3. Joseph Benavidez

If it weren't for Cruz, Benavidez would arguably be at the top of this list, as the current champion is the only fighter to defeat him during his 18-fight career. In August 2010, Benavidez battled Cruz to a split decision, but he came out on the wrong side and wasn't able to earn another title shot against a fighter he'd already been beaten twice by.

4. Brian Bowles

In 12 fights, Bowles has only lost against current bantamweight champion Cruz and a fighter who could eventually find a spot on this list in Faber. He was unable to defend his WEC title against Cruz, but Bowles earned the belt by dethroning one of the greatest bantamweights ever in Torres.

5. Chase Beebe

Before Torres rose to the top of the bantamweight division, Beebe won 12 of his first 13 professional fights and defended the WEC title against Rani Yahya. While the impressiveness of Beebe's short run as champion will likely be erased as the 135-pound division continues to evolve, he deserves a spot among the division's all-time greats for now.