There are so many threads on this already... but I'll try to summarize for you.
High tier (characters you can go AFK with for botting):
DK's obviously can sustain themselves, have a pet to tank mobs, self heals, high armor, high damage... and death grip pull
Warlocks have their pets take damage all day so you literally need no gear to bot with them, cheap route, very fast levelers.
Paladins are pretty much same as Dk but a little slower because they have no pet or pull
Mid-tier (average sustained xp/hr):
Hunters become pretty good after you gear them up a bit and they get more skill/pets. Can be as fast as warlocks but more gear dependent
Any hybrid class with heals (druids, shamans priest)
Low-tier (very situational xp/hr depending on profile)
Warrors, rogues, mages
Low-tier
