My party at the moment (and probably for the rest of BG I, and most of BG II) consists of :
Main character - Human Sorcerer
Imoen, Human Thief
Minsc, Human Ranger
Dynaheir, Human Mage (Evoker)
Reasoning behind all this :
Sorcerers rock. Nothing more to that choice, really. It wasn't possible to play a Sorcerer in the original BG I, this only works because of the BG II / BGT thing. It does mean that I can choose any mage spells on level-up - I don't need to hope I find the right scroll in the right place. Also, making the Sorcerer the main character (first created) means I can cast Find Familiar from a scroll and get bonus 12 hp early one - essential to survive as a magic-user !
Ranger/Cleric - they rock too ! Able to wear any armour, and cast any Cleric or Druid spell (even the higher-level Druid spells, which a Ranger normally wouldn't be able to cast). The only drawbacks are slower levelling due to being multi-classed, and no sharp weapons - but there are enough good warhammers/maces/flails in the game (especially once we get to BG II) that this isn't a problem. Also, there aren't any Ranger/Cleric NPCs, so if you want one, you have to create your own.
Thief - there's no single-class Thief NPC that you can pick up and play through the whole game, therefore if you want one you need to bring your own ! Theives are always useful for trap-disarming and back-stabbing, and once they get the high-level abilities and special traps they're near unstoppable. Half-orc, just for a change ...
Imoen - got to take my sister along, right ? And two thieves means I can spread around the skill points - Imoen gets open locks and hide in shadows, the other thief gets find traps and set traps. Eventually someone will get pick pockets ...
Minsc - he's funny, and he's a decent "tank", and is needed to keep Dynaheir happy.
Dynaheir - because a sorcerer can't handle every situation, you need a general mage along too. And with her and the sorcerer blasting away with damage spells, few opponents stand a chance.
How did I get three characters I created myself ? Simple - I started a multiplayer game, where you can create 1-6 characters at the start, then save the game at the first possible point. Copy the saved game directory from "mpsave" to "save" under your BG installation, and you can load it up in single player with all of the characters you've created. Very handy !
Apart from a small technical hitch where the bridge to Baldur's Gate didn't open up properly (possibly because I somehow managed to do Cloakwood Mines and Nashkel Mines out of order), everything has gone pretty smoothly. Then again, I'm not just the player in this sort of game, I'm also the DM, and I get to nudge things in the right direction if things do go too far awry.