Honestly though I really gotta give Honorable Mention to Karlach. It's so rare to see a character so genuinely positive and upbeat about everything, doubly so when they're as openly passionate in expressing their thoughts and feelings as her. The tragedy of her circumstance would have been so much weightier if it were earned by the player's own poor choices or failures instead of just being inevitable.
+1, I don't actually have one favourite character since they were all part of any amazing story. My Tav didn't get along with Astarion until Act 3, but that's a problem between those two. I also didn't think Halsin is nearly as bad as some people felt he is, but in terms of team dynamics, Karlach was so important in my playthrough. All of the companions have their own inner and out battles to fight, so I could imagine that many camp nights and dungeon crawls would have been rather gloomy with everyone staring holes into the campfire and fighting spectres within and without, were it not for Karlach.