The spell caster is likely the strongest among these 4. Being able to instantly learn any spell is almost cheating. Among them would probably be a spell to temporarily switch to any other class. Overlord did it.
@Vladimir Efest With Caster you can only use elemental magic so there wouldn't be a spell for a class change. Overlord was awesome though. Btw all i did was look up "Asura's Wrath Asuras 6 arms" and found that on pinterest, it's just a picture from the video game