While above the guy said not everybody plays blood DK, I suggest it. Their whole entire class revolves around self healing and survivability, which gives the bot a lot of leeway to mess up in fights. Plus, deathknights start at level 55. I'm not too sure why anyone would play anything other than a blood DK. Even for farming, druids may be a bit better in that they don't have to dismount, but if you're on a PvP server then my DK with 800k HP isn't going to get attacked, while a druid with 500k just might. Not to mention that Blood DKs can solo just about any instance that isn't impossible to solo in this expansion.
Blood DKs are overall the best botting class.