The most useful monster.

OK. Here are the answers: Angel, Magic Dragon (nature), and Djinn. (This trio won't have very good morale, though). Now about Black Dragons and Water Dragons. Black Dragons will be defeated, just like all melee units, and considering their high cost, this is expensive, and you can't "bless" them. Of course, 10 Black Dragons will defeat 10 Water Dragons, but I would prefer to fight with a hero + 8-10 Water Dragons rather than Black Dragons.