This question is next to impossible to have an agreed upon answer due to the subjective nature of the word great. But with that said I'll give it a shot. For the sake of added excitement, I'll try to stick primarily to title matches, since those have the benefit of added drama due to the belt being at stake.
WSW: Joanna Champion vs. Gadelha II
WBW: Holm v. Tate
FLW: Johnson v. Dodson I
BW: Cruz v. Dillashaw
FW: Aldo v. Mendes II
LW: Edgar v. Maynard II
WW: Robbie v. Condit or Robbie v. Rory
MW: Chael v. Sonnen I or Weidman v. Silva I (be honest, it's probably one of the most memorable)
LHW: Jones v. Gustafsson
HW: Couture v. Sylvia. For a 1-rounder maybe Stipe-Overeem?
But you see, even in that list I had to judge each choice on wildly varying criteria. There's almost no equivalent to Jones Gustafsson in Heavyweight because the HW title mtaches are almost all completely one-sided. I could pick Rutten/Randleman I guess, but that fight wasn't visually as exciting. Plus as good as Hunt/Bigfoot was, there weren't really any stakes involved, so can you say it was a more significant fight than a title match?