dunno why people are saying Pride 25, granted it was a great event but best ever?
I'm most likely bias because I went to it live, but Pride 33 somehow still stands out to me as one of the greatest events in MMA history. High level match-ups, lots of finishes. Nothing but excitement in every fight.
If not that, I'm going with Shockwave 04. The combo of Hunt/Wandy and Fedor vs Big Nog to me makes it better than any event ever alone, add in Gomi vs Pulver, Chonan's flying scissor heel hook of Anderson, Decision Dan stealing a victory from Yuki Kondo, it was all and all, an amazing night.
UFC 100 was amazing, whatever the Bisping vs GSP card was too delivered in a big way. UFC 84 (BJ vs Sherk, Wanderlei vs Jardine!)
UFC's from 43-52 were all epic.
I don't know all the Strikeforce #s but they had a few really amazing events as well. Dream had a few that were just crazy good.
nobody has mentioned any but, the k-1 Hero's grand prix's were nuts too.