The most useful monster.

I would probably agree that a vampire is one of the most overpowered units, and the situations where a vampire is useless are rare. I had a situation where vampires were defeated by magic and hydras – the vampires didn't deal any damage in return and didn't have time to regenerate. Then I replayed the mission and used poisonous creatures – they destroyed the hydras standing on the city tower.

I would put djinns in second place, unless the opponent has high magic resistance.

And I would put cyclops in third place. It's very advantageous to deal high area damage and not take return damage from enemy archers.