HK-47: Of course. This guy's just a laugh riot, even when playing as a light-sider. He was pretty meh in KOTOR2, but then again so was just about everything in that game.
Tali: Just plain adorable really. Probably the only romantic interest in a Bioware game whose romance subplot I've enjoyed.
Garrus: Space Batman with an awesome voice and some delightful sarcasm.
Varric: Best character in a rather mediocre game. He's snarky, practical, and good-natured, and was BFFs with my own sarcastic Hawke.
Kang the Mad: I like the crazy ones. I especially like his suggestion for causing someone an accidental death, say by "falling down a flight of punches".