There's too many to remember / choose from, but one that springs to mind immediately is UFC 84.
Coming off of 2 nasty KO losses and the hardfought but sound defeat to Chuck, seeing Wanderlei just obliterate Jardine was about the most excited I had ever been for the finish of a fight.
And then there was BJ looking unstoppable against Sherk, Machida beating Tito, Stun Gun and Yoshida getting awesome victories, Thiago with a brutal stoppage, Shane Carwin, Goran Reljic..... it was a hell of a card top to bottom, one of those nights where everyone was having a great time and the drinks went down easy.