Another vote for Cain vs Nog, also would say jones vs bader, after that fight I was convinced as they were both unbeaten at the time and thought bader had a good chance prior to the fight. 
SupesUp - Rashad vs chuck
Was gonna say this 
Mir vs Brock 1 brought Mirs career back from the brink after he couldn't seem to get it back together after the bike accident
Penn vs Hughes. No one expected Penn to rnc Matt at that stage of Hughes career.
Joe Lauzon Vs Jens Pulver
jimbonice -Penn vs Hughes. No one expected Penn to rnc Matt at that stage of Hughes career.
Fuck! Came in here to post this and thought I might be the first. Easily Bj beating Hughes at UFC 46, nothing else to me even comes close. VTFU though for being quicker than me.