i understand that the ufc has an avenged losses dvd out (which i have not yet seen)... it will be interesting to see how they identify and rank the fights... here's my take:
eleven: b.j. penn - matt hughes... sorry but this has to be on the list but i have this non-title ufc 123 knockout of matt hughes, avenging the ufc 63 loss with the welterweight belt on the line at number 11.
ten: georges st-pierre - matt serra... i'm so conflicted about the significance of this one. this avenged loss does not seem all that huge given gsp's total dominance of the welterweight ranks over the past several years. but of course there's this little thing called a championship and with all of the talk and such leading up to this second matchup, it indeed was a big deal.
nine: frankie edgar - gray maynard... reluctantly on my list but the fact that the lightweight championship was on the line makes this ufc 136 avenged loss undeniably significant, even if the hard fought battle was strangely unsatisfying.
eight: chuck liddell - jeremy horn... it was the iceman's first title defense at ufc 54 in which he avenged the ufc 19 submission loss to horn. jeremy hand handed chuck his first pro loss by submitting him with an arm triangle choke.
seven: anderson silva - yushin okami... in the main event of ufc 134 (the ufc's return to brazil) silva avenged a dq loss from their january 2006 'rumble on the rock' bout in which anderson delivered an illegal upkick that cost him the fight.
six: mauricio rua - forrest griffin... forrest shocked the world (or perhaps it was just myself) when he submitted shogun in ufc 76. so it certainly must have been sweet to crush forrest in front of his countrymen at ufc 134 (rio).
five: chuck liddell - randy couture... the iceman's second mma loss was to randy at ufc 43. it was a tough 3rd round tko as the two fought for the interim ufc light heavyweight belt. but chuck avenged the loss by knocking randy out during their ufc 52 rematch and took the championship with him. he also ko'd randy in his ufc 57 title defense.
four: brock lesnar - frank mir... in ufc 100, brock brutally avenged the very high profile and emotionally painful submission ufc 81 loss to mir. brock went on to taunt mir immediately after the ufc 100 win upsetting fans and a key ufc sponsor while doing so.
three: dominick cruz - urijah faber... cruz avenges the one-handed wec 26 submission loss this ufc 132 bantamweight fight that was so good that both fighters were awarded 'fight of the night' bonuses.
two: georges st-pierre - matt hughes... huge because this ufc 65 title victory over hughes was not only avenging their previous ufc 50 contest in which hughes submitted gsp, but also because he had finally overcome the man who was a extremely dominant welterweight champion.
one:
mauricio rua - lyoto machida...

i rate this as the most meaningful avenged loss because in their first meeting (ufc 104), the three judges completely and unanimously screwed up the decision awarding the victory to machida. this resulted in lyoto retaining the light heavyweight championship when it clearly had been earned by shogun. the decision was so bad that the ufc set the two up to fight again which they did in ufc 113, in which shogun decisively defeated machida, knocking him out in round 1.