That's a good question.
As a practical matter, it is almost certain I will end up railroaded into the "Dungeon Master" class.
I like most of the classes I actually know something about. I don't know much about rogues, clerics, or rangers though, so I can't really offer an opinion yet on whether I find them personally appealing.
I'll probably do my usual of coming up with an idea, then building it. I've been wanting to play a halfling fighter with a machete for some time. I was going to use Tome of Battle and the Tiger Claw school, but never got the chance. Maybe I'll give him a whirl in 4e. I know its probably suboptimal, but I've never minded much.