My rating:

1. Vampire.
I wanted to put a Genie here, but it's already leading with a huge advantage. Besides, with a hero having Tactics, Vampires move before Genies, reach them..., and basically that's it - they kill them. Although, in small numbers (at the beginning of the game), Genies are still stronger. And they are definitely more useful in battle, so I cheated a little in the voting.

2. Genies.
No comment.

3. Black Dragons.
They will annoy any army, and against Haven (including Genies) - they are irreplaceable.

P.S. I'm not considering various monsters from add-ons and mods, as I haven