So I know that this game is farely balanced, nevertheless i think there are a few premium builds, feel free to discuss, to add to or remove any of my suggetions:
1. Control Wizard - has been strong since release, adding wizards tower just made this build go through the roof!
2. Beastmasters Swarmimals - His ability to cast additional animals is super strong and will bring you a lot of troubles! Tanglevine the mage and hunt him down!
3. Curselock - Curses are VERY strong in masses and the male warlocks ability to recycle them is badass!
Honorable mentions:
Adramelech warlock can be quite awesome since she may try a demon swarm with wildfire imps or just praise the lord of fire and go for the throat! Had a lot of fun with her and i see her as being a very good mage!
The Forcemaster might be good too, but I am just not good enough to play her and there are many mages that play nonliving creatures, which are not affected by sleep
Necromancer may swarm good too and can choose between skellies and zombies, which I love, but he is missing an alternative winning option (Sardonyx is not a threat x.x)
I have not played the druid yet so i do not know how good she is and what she is able to do but she seems too slow to appear on this list, furthermore she has a big disadvantage since she is super vunerable to fire!