I usually pick them like this:
Nature
tigers (might seem strange....but...I like them more than archers)
unicorns
magic dragons (magic!!!)
Order
golems (mages are killed very quickly, and their magic fist isn't that strong)
djinn
titans/dragon golems (I can't figure this part out at all. Both are so good that it's hard to choose:confused: )
Life
ballista
monk
champion
Chaos
medusas (shooters+spell)
nightmares (because of terror)
black dragon (: ) :) :) no doubts here);)
Death
cerberi (stronger, more useful)
venomous spawn
devil (teleport to any point in the battle:cool: )
Barbarian
harpies
cyclopes
thunderbirds