Best solo heros in the game atm are wizard and barb (wizard coming way ahead)... DH is good for solo too but there are situations that DH can't manage but wiz + barb can... WD can solo but it's harder than with other ranged guys
Monk is the 1 that can't really manage later acts (you can with decent gear but you can't really farm the gear yourself)
Ranged: Wizard > Demon Hunter > Witch Doctor
Melee: Barbarian> Mushroom > Monk
Thanks for the reply. What about PVP-wise? Is Barb and Wizard viable to be strong?