The Coleman fall has already been mentioned , but I'll quickly add a few of my other favorites. Every single one of these things made me laugh fro awhile the first time I saw or heard them.
The Arlovski /Datsik fight( AA's first) - where Bas and Quadros are laughing uncontrollably as Datsik is going Van Damme , with some crazy movie type stuff that never works. Funniest part is he actually won ended up KOing Arlovski.

Joe Son getting repeatedly nutt punched by Hackney
Rampage saying he is going to root for Wandy to get a quick KO , so he can take a s**t.
In the 8 part cro-cop documentary -when the old guys get in the drunken back yard fight. When one gets tagged Mirko lets out this high pitched laugh that was priceless.
Luke Cummos contemptuous front kick at the end of the fight against Torres on TUF 2. It was like a exclamation point of ownage.
Bob Sapp sitting down and crying after Mirko popped him.
Bob Sapp just picking Big Nog up and setting him aside , while Nog was trying to get position on top of him
Burkmans choke throw on Ficket into the side of the cgae
Tito underhooking Shamrock and handtrucking him across the octagon
Almost anything Tank Abbot has ever said.
Matt Hughes just shaking his head and saying " Frank" ,as Trigg gives the most obvious attempt to hype a fight in history. He say stuff like" I come from a better family, I'm better looking, I have better kids" It was hilarious.
Hughes parting shot after Serrras stormed off after a rant about Laimon not being a fighter when Hughes commented how he would like to see them roll together. " So are you gonna roll? "
Wandy proving his English wasn't quite good enough yet" I want to F**k Chuck"
Wandy to Page" I'm going to make you my new Sakaraba"
Rampages video where makes Congo cracks about Wandy on his old website while playing video games.
The look on Eddie Sanchez's face as Mirko stalked him around the ring
Forrest apologizing for spooning Mahood
Houstons choke slam on Silva
Travis Lutter ( of all people) landing a one punch Ko on Eastman to start the second.
GSP incorrectly correcting himself. " I beat him square and fair ... Oh .. I'm sorry.. fairily. squarily" '
The entire Diaz/Riggs episode on Sherdog radio.
Almost anything Mayhem says
The slow-mo shot of Dana's face watching the ping pong ball in the Pulver/Penn cash game.
The all time greatest for me is in Manhef /Cyborg. That fight was such an incredible war that when Manhoef won, Cyborg didn't know. He sits up and he is still trying to hit a exhausted, victorious, flat -on -his back, collapsed Manheof. He throws these little hammerfist that look like he is trying to give CPR.