by "level solo" i'll take it as : solo elite quest mob(aka. 2~3 group quest) plus the long grind with little down time.

i would say
demo warlock, hunters, affliction lock,
aoe, one by one, solo group quest, little or no downtime, all good
you only stop when either you ran out of quest mob, or your eyes are tired.

next would be

feral druid,
one by one, solo elite, no downtime.

frost/arcane mage, Boomkin(balance druid),
master of aoes, but need to eat or drink.

i'm not sure of, but they maybe good but with some downtime:
enhance shammy, paladins, rogue(maybe)