The best solo class IMHO is a Demonologist Warlock. You never have to buy water. If you're careful, you can keep most of your health in a fight, use Life Tap afterward, then restore the lost health by eating food.
HOWEVER, if you're really good with the Warlock and have the right talents, you can lose only a tiny sliver of mana during each fight.
If you have a lot of stamina, simply put Curse of Agony, Immolate, and Corruption on the mob you're fighting. Do it slowly, not generating much threat. While your Fel Guard tanks the mob, use Life Tab a single time, then use Siphon Health on your victim. You'll come out of the fight with full health, and only 1/30th of your mana missing. This will re-generate within a few seconds.
If you keep this up flawlessly, you can fight an unlimited number of mobs in a row without stopping for a food break, and never being too low on resources. I'd buy up to a stack of 10 food if you're using this strategy, in case some disaster happens and you lose more health than you should. I've just started using this strategy, and have been leveling MUCH faster than normal.
TL;DR: Demonologist Warlocks are best.
