my personal favorite is the monk. but it all depends on your style. I'm not really a fan of multi-classing. you just cant level-up fast enough. I've never played a sorcerer, i know what they're abilities are, but ive never played one. I'd stick with a straight mage though. they can memorize anything, and they are not as limited as a sorcerer. Baldur's Gate 2 is more geared towards good-aligned parties. if you are good, the game generally flows better. BUT, being evil is a blast. It's just that the game is more focused on good. which is unfortuneate for evil parties out there.
So, I would play a monk. But my opinion really doesn't mean a whole lot, play whomever you feel like you would have the most fun playing, depending on your playing style. have fun!