Whichever you feel most attracted to. You'd have to play the lot to experience all the changes.

My feeling is that cleric is the easiest pure to get to grips with, and warrior the easiest square. I'm not sure how much change in bards and rangers around the changeover but I think they are the least changed in the last 10 years. For UAC you should play a monk, but I'd recommend getting used to the general game with some other profession first before trying out UAC.