Hard to answer considering no one can immediately recall every good bout they've seen then rank them accurately. Some like sloppy gassed out rock 'em sock 'em robots bouts. I like when something awesome happened like some great technique executed flawlessly, or one guy rising to the occasion and being head & shoulders better than his opponent.
In no order:
GSP vs Hughes 2, Fitch, Trigg
Hughes vs Trigg 1 & 2
Jones vs Shogun, Lyoto, Vitor
Crocop vs Wanderlei 1 & 2, Aleksander Emelianenko, Igor Vovchanchyn, Magomedov, Herring
Gomi vs Azeredo 1
Mark Hunt vs Bigfoot Silva
Matt Brown vs Erick Silva
Niko Vitale vs Jason Miller
Rampage vs Wanderlei 1 & 2
Fedor vs Randleman, Coleman 1, Fujita
Eddie Alvarez vs Tatsuya Kawajiri, Joachim Hansen, Katsunori Kikuno
Pat Curran vs Marlon Sandro
Nate Marquardt vs Tyron Woodley
Josh Thomson vs Nate Diaz