I composed this list of what I think are the biggest MMA upsets to date. Its purely on my opinion so don't get mad if your favorite fighters aren't here.
10. Joe Lauzon vs Jens Pulver-The two fighters met at UFC 63 with Jens being the favorite as the first ever UFC lightweight champ. This didn't hinder Lauzon though, who knocked the ex-champ out in 47 seconds with a huge left hook. This was Lauzon's UFC debut.
9. Anderson Silva vs Hayato Sakurai-The two fighters met in Shooto, Sakurai being the undefeated champ and having defended the belt 7 times, with Silva the underdog. The fight went to a decision and Silva claimed the Middleweight bout in only his sixth fight.
8. Kevin Randleman vs Mirko Cro Cop-In the opening round of the Pride 2004 HW GP the two fighters met with Cro Cop labelled as the favorite with his devastating knockout power. The tables were turned though as Randleman knocked Cro Cop out cold in the first round with some brutal ground and pound.
7. Randy Couture vs Chuck Liddell-At UFC 43 the two fighters met. Couture at 40 was the underdog. Feared striker Liddell was expected to walk through his opponent but from the beginning Randy controlled the fight winning by TKO in the third round after some heavy ground and pound. This bout was for the interim UFC light-heavyweight belt.
6. Frank Mir vs Antonio Rodrigo Nogueira-UFC 92 was the venue for this fight. Big Nog had never been stopped in a fight and was heavy favorite over Frank Mir. Nogueira who was the first Pride HW champ was schooled by the underdog Mir and in the second round suffered his first TKO loss to the submission specialist Mir after some punishing punches which floored Antonio. This bout was for the interim UFC heavyweight title.
5. Gabriel Gonzaga vs Mirko Cro Cop-The two met at UFC 70 with Mirko Cro Cop the heavy favorite against relatively unknown Gabriel Gonzaga. The upset came in the first round when Gonzaga knocked Cro Cop out cold. An even bigger upset maybe was that it was with Cro Cop's own trademark high kick that he was himself dispatched by.
4. BJ Penn vs Matt Hughes-The two met at UFC 46 with the Welterweight champ Hughes a heavy favorite against BJ Penn. Penns's record stood at 6-1-1 coming in to this fight and against Hughes' 35-3 many considered this a walkover for the champ. BJ didn't see it the same way though and in the very first round he won the title by way of rear naked choke on the submission specialist.
3. Randy Couture vs Tim Sylvia- The two met at UFC 68 and the then champion Sylvia was a heavy favorite with an age, size and weight advantage on his opponent Couture. From the first seconds of the fight it was clear that things weren't going to go Tim's way. As soon as round one started Randy floored tim with a big punch, he then took Sylvia down hard and pounded him pretty much the same way until the end of the fifth round. The 43 year old had beaten the odds by dominating the giant champ for 25 minutes and reclaiming the UFC heavyweight title.
2. Remeau Thierry Sokoudjou vs Antonio Rogerio Nogueira-The two met at Pride 33 and Sokoudjou was 16-1 underdog in only his fourth professional MMA bout. Rogerio's record stood at 12-2 coming in to this fight with wins over K-1/Alistair-Overeem">Alistair Overeem twice, Dan Henderson and Kazushi Sakuraba. Many believed that Sokoudjou stood no chance. In the very first round after 23 seconds Sokoudjou knocked Nogueira out cold with a left hook.
1. Matt Serra vs Georges St.Pierre-The two men met at UFC 69 and the Welterweight champ GSP was heavy favorite to win. Serra who had won TUF season 4 was 11-1 underdog to take the belt. In the first round Serra rocked GSP with a huge punch which downed the champ, Serra kept his distance waiting for his opponent to rise before again flooring him with big combos. Eventually Georges was floored and couldn't rise as Serra pounded his head against the canvas,the ref stepped in and stopped the fight with Serra the new champion. He was the first and only man since to knock out St.Pierre.
Some info sourced from Wikipedia