If its just DPS go for mages, unless like me you can't bear them

'Locks and Hunters perform well, personally I find Hunters to be more involving, a lot of what my 'Locks do is dot -> dot -> dot... next target. And at 12 I can just about 'solo'* a level 19. *(thats really a pair but I'm playing on my own so yes solo ).

Hunters on the other hand certainly at low levels need a more involved play style and, IMO anyway, are a bit more fun. Plus if you pick the right pets and abilities (thinking Cats/Ravagers) you can get quite a big boost to DPS, or going with Pig/Bear/Turtle you get a reasonable off tank for non-heroic anyway. Plus of course the CC from traps which can be very handy.

I would think from a multi-boxing point of view the only classes that wouldn't be that effective would be Warriors and Rogues, because melee which is 90% about position is a bitch when two of three are just following.