"You killed Ug-Ug, my only friend!"

"Ahh, cyborg!" (Only when spartans show up)

"Run away!" (When the elite/brute next to the group dies.)

"Me kill you!"

"Grenaaaaade..." (Boom)

I'd check out the effects of the IWHBYD skull in Halo 3 too. They say some pretty funny stuff there.