If we approach the question of usefulness from a different (and debatable) angle, answering the question "Who would I NOT want to see in an EQUAL enemy army?", here is my top three of opponents. (from Fan of Nature)
1. Venomous creatures (they simply live up to their name)
2. Cyclops alongside catapults (sometimes, I simply run out of words from frustration)
3. Gorgons, or mad piglets (I don't remember what they're called, those that run around on their own and magic simply doesn't affect them)

And here is my (subjective) top three of usefulness

1. As much as I don't like them, I have to give credit to DJINNS (when I have the honor of playing WITH them, I always place one Djinn in every available space, so that the opponent doesn't have an easy time)
2. It's a pity that there are no more detailed clarifications; I will choose usefulness for combat (at the beginning of the "big game," it's definitely Cyclops, and then Phoenixes and Locusts). I will probably give credit to my favorite MANTISES.
3. I would put THIEVES in third place (you can't underestimate their usefulness at the beginning of a difficult map for rapid development; as sad as it is, money is very useful)