Manticores – just a beautiful unit, but I don’t have much to say about them because I haven’t played with them.
Phoenixes – no comment! Resurrection, the biggest initiative in the game, is simply terrifying.
Death Knights – specifically they, and not Black Knights; Black Knights do not have half damage.
And I generally don't know how to make fire birds! You can only place Black Knights in the editor, as well as Yetis; there is even a special feature in the editor with a whole lot of cool things (read this topic), but I didn't see them there either!
The Elementals turned out to be somehow bad; I don't like them.
Mummies are also bad, but until they start messing around with resurrection.