Magistar
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You also need to consider the potential of the units, and Nightmares and Hellhounds have almost the same potential. The difference is that one scares nearby and the other attacks, both at the beginning of the turn.
Archdevils are good, especially when playing against Necropolis, not counting the speed advantage over Archdemons. Destroying corpses prevents them from being resurrected, it's like a substitute for the hero's Corpse Consuming ability, which is more useful than a hook.