The most useful monster.

because angels with resurrection (when a hero-mage was killed in the final battle, and he didn't have time to drink the healing potion a second time)

For me, it seems a bit strange to determine the usefulness of a creature based on the final battle, although this is not entirely meaningless. But first of all, you need to think about development, and preferably fast development, otherwise you might not even reach the final battle in the sense that I understand it, if the opponent turns out to be more agile, I mean online, and in offline mode, there is also nothing to do without fast development of the economy or the hero.

Added after 8 minutes

P.S. I also like Djinns, but since they can catch up with Vampires, I won't vote for them))))

Djinns are good a little later in the early game, when there are enough of them to summon a large number of illusions. Then, for battles, it is enough for the hero to take only Djinns and the Original for the illusion. Four slots with one Djinn each for "songs of peace" and in the fifth slot, put all the rest to copy illusions from the Original. And in the seventh slot, the Hero himself :)