No doubt in my mind - it's Jan, Jan, the Turnip Man!!
Not only can this weak, mouthy little gnome make his own ammunition, but it is the best in the game - it stuns the baddies, letting my fighters hack them to pieces. And he has literally made me rich with his pickpocketing (which almost never fails) - after I paid my 20k GP at the end of Chapter 2, I still had around 150k (and haven't found anything to spend it on yet). He is flawless in detecting traps and picking locks, too - never failed once for me. Sure, he's weak, but he's not a melee fighter anyway, and there are various items that can remedy his weaknesses.
Also, by the end of Chapter 2, he had gained enough lore to identify items.
On top of this, he's a mage!!!
The only serious weakness that I can think of is that he has only one attack per round.....
Great personality, and I kinda enjoy his stories.....
No better teammate in the game, IMHO.
JaggedOne