Of course, the Supermutants from FO3, and anything said by the raiders in Borderlands...
But for my personal favorite: anything said by Jo' Momma (Oddworld: Stranger's Wrath). Gotta love a villianess that shoots her own minions! Seriously, that game did not get the respect it deserved... so much hilarity ensued from just about everywhere, even while just talking to the random townsfolk. Also, the random screams coming from the grunts is pretty darn funny too, especially when they get nipped in the butt by the Fuzzles.
On a semi-related note, although technically not enemies, the Chippunks are just downright hilarious; they'll even insult you while they're waiting to be fired XD