Although summoners classes were a bit unbalanced in beta, they seem to be working on this. Either the Sorcerer or Summoner branches are both good spell-casting solo'ers. Warlocks, Wizards, Necros, and Conjurers should be pretty good since they all get some great nukes, dots, and slows/roots. Enchanters however, are much more mediocre at damage and are really stuck in groups for the most part (but they could solo I just find the others to be much easier to).