Though I can't find the link, I'm almost sure we've covered this before, but not with the changes after the expansion.
Reader Will dropped a tip (thanks!) saying he has almost reached 70 with his Hunter, and really enjoyed playing his way up because Hunters are so good at soloing. I have to agree-- I haven't played anything as relaxing to solo as a Hunter, just send the pet, throw autoshot on, toss in a few arcane shots or stings for good measure, loot and repeat.
His guildies say if he rolls a new alt, it should be a Rogue. Personally, I think Warlocks are a great class to be a solo artist with, but Druids certainly have been making headway lately. Will also wonders about builds-- I've had a great time soloing with my shadow Priest, but I've heard holy priests are a pain to solo (my resto Shaman has certainly been more annoying than a few other alts I've had).
Because Blizzard has done so much to make almost every class viable solo, I'm sure everyone's got an opinion. Share yours and help Will out here.