I'd go with Stan for favorite charecter, although I like Kyle a lot as well.
Both of those charecters seem to anchor the show, and keep it from going too far away from reality (relatively speaking-it's often far from reality as we know it, and that makes me glad!).
Butters is a close second favorite. I've known people, both kids and adults, who were that naive, sometimes willfully so. I like watching Butters use the bathroom (that sounds really bad, dosent it?). The way he drops his pants to the floor to stand at a urinal asserts his uber-childlike qualities. Seriously, three year olds do that. Eight year olds don't.
I like Kenny too. He's underused, but when he is used, he seems to be very good hearted. Stan and Kyle are the moral and intellectual center of the show, but Kenny is it's heart. Cartman is it's anus.