fighter/mage/thief
or
fighter/cleric/mage.
you never get as powerful as you would just specializing, but I always tend to balance out all my stats. I mean, it ain't just in baldur's gate - in morrowind, oblivion, and, well, shit in every single game I've been given the choice I almost always end up trying to be good at as many things as possible. I pretty much solo'd the BG series as a fighter/mage/thief.
One thing is that I'd rather not have to choose at the start what my class or skill sets are or should be, but rather like fallout or morrowind, choose some basic stats and then be allowed to develop them in any way I want, not just down one set path.
oh, and always a gnome or dwarf (or just generally short, if possible). Don't know why, just . . . well, I like to role play short people, alright?