The 'Don't Waste Your Money on This Dross' class beats all the others.
Not much I can say to this comment other than "Why did you bother

..." - pity there isn't a troll class in D3 ...
Back on topic ...
I'm a complete altoholic when it comes to any game where you can have multiple toons like this and at the moment I've got one of everything on the go except the Barbarian.
Wizard - have only played this one co-op witih a Barb and it starts of pretty weedy, but put some +int gear on it and it's great for nuking from range with something like Arcane Orb, although mine was also doing fine up close last night with Spectral Sword
Witch Doctor - have been playing this one this morning and having now got the Zombie Dogs, it's gone up a gear as I can just stand back and use the blowpipe from range. My bro has one of these around level 20 and he assures me that the damage he puts out is absolutely insane
Monk - this is probably the most therapeutic to play, particularly if you're still using fist weapons! The sheer attack speed is fantastic and again, I'm assured they're absolutely mental at later levels
Demon Hunter - probably the one that puts out the most damage at lower levels, in my experience at least, particularly when you get a crossbow in each hand and the rapid fire skill ... entering a room and hosing down the mobs is awesome fun, although it absolutely chews through your hatred
All mine are low level - the wizard is the highest at level 11 - but all are so much fun, the hardest decision is deciding which I want to play when I sit down for a session.