It really depends what content you want to solo. There's a thread on this forum listing what warriors have been able to accomplish. As a general rule, you can solo anything that either won't kill you through second wind (3% health per second) or anything you can get lots of victory rush procs on (non-trivial npcs).
I think your best bet is still probably a death knight depending on what you want to solo.
If its old content where your vengeance isn't going to get particularly high such as cataclysm, then yes a death knight.
If its current content such as the guy that soloed galleon, you'll probably want to go for a paladin because of how beautifully they scale with vengeance in terms of damage and healing.
Then there's monks, a monk isn't really regarded as a solo artist depending on what you're talking about. Stone guards hc for example, monks are generally about the avoidance and reducing magic damage by a lot, meaning fights like that are easy for a monk.
It boils down to paladin vs monk vs dk, depending on which you like best, don't go warrior though.