The tiers are pretty much...
Bots, thugs, demons
Necro, mercs
Ninja, and last beasts.
Any of these would be fine if geared properly. Problem with beasts is its all melee. There defenses dont make up for always being in melee range. The top 3 all have good offense, and good defense. Bots after you get the final power is pretty amazing. I'm working on a demon now and so far it's pretty decent.