Pretty much everything in Portal. "Didn't we have some fun though? Remember when the platform was sliding into the fire pit and you were like 'NO WAY' and I was all, "We were just pretending we were going to murder you?" That was great."
The plot twist in Jak 3. I mean... wow. That was freaking hilarious, but it worked with the story too.
Lots of lines in Mass Effect 2. Shakespeare performed by Elcor, the vid about the first Hanar spectre, the speech by the commander... win.
Lots of moments in Halo multiplayer. That game was made for some of the awesome stuff that goes on.
EDIT: Also, that one Pokemon trainer in the first game. "Hi! I like shorts! They're comfy and easy to wear!" Then he battles you. That's tied with the first Final Fantasy's "I, Garland, will knock you all down!" for the best battle cry ever.