In my opinion, it's the Sylvan faction, because they can easily learn light magic, have fast leveling, and strong units. Also, they can choose three Necropolis units as their sworn enemies. The hero is Ossir (I think everyone knows about his advantages).
The thing is, I have a battle against a Necropolis player coming up soon, and I don't want to lose. The opponent's hero will most likely be Kaspar, and there will be 3 other computer-controlled players.