I like racing muscle class cars, mostly because how fun they are to drift around (though I tend to spin out a lot.) Not a big fan of supers, mostly because I dislike super cars in general, but eh. I'd have to say sport classics are a close second though.
Since the new breed of sport classics, I think they're the most fun to race with. There's also some variety unlike supers and sports with the Vagners and the Pariahs all the time. Least favorite would be bike races because of the ridiculous wheelie mechanics. I'm not a big fan of the compacts class either. The Brioso spins out on everything. I hate that ice skating piece of crap.