Warlocks are pretty strong solo as they function somewhat similarly to a hunter in that they have pets that can tank while they deal damage from a distance. They also have abilities to restore health/mana so not as dependent on food/water and they also don’t have to keep their pet happy by feeding them or concern themselves with running out of ammo for their ranged weapon.
Druids are a very strong choice eventually. They start a bit slow but ramp up a bit at 10 when they obtain bear form, again at 20 when they obtain cat form and considerably more at 40 when they obtain dire bear form. So you can tank damage if need be, you have heals at your disposal, and the ability to stealth around in cat form should you want to get into and out of a location on the sly (specifically, if you don’t want to fight your way all the way to back of a cave just to pick off a single target or acquire an item), druids toolkit is truly jack of all trades.
Edit: I might also include rogues. Same stealth as a druid, more damage, ability to vanish if crap hits the fan, evasion if you happen to pull a bit more than you wanted and vanish is on cooldown.