SilkTopHat: None. I never play with the pre-existing NPCs; I always generate my own and control the lot of them. I'm sure I'd have a different experience and come to like a few of them if I used them, but the fact that they argue with and kill each other is not enjoyable. Personally, I find it less immersive if I have to play babysitter than if my party members are robots.
you miss a fair bit of story if you don't take some NPCs - if you don't want them fighting then either use a mod or just pick characters of similar alignment (aforementioned BG1 team of PC, Jaheira, Khalid, Imoen, Minsc, Dynaheir won't fight at all).
NB: I've only played through BG1 once (used that party) and am currently on my first try at BG2 (also with a team that don't fight each other - Minsc, Jahiera, Aerie (romance clash but you can stop it by only going for one), Imoen, Keldorn)
Not sure I'd say any are indispensable though - if any, I'd say Imoen for story but if/when I replay I'll try an all-evil party and not take her along. Not sure who I'd have for BG2 in that case though.