Which monster do you like the most in Heroes 5 at level 5?

Hydras don't even need speed and initiative

And hydras are precisely indispensable so that the hero can use their spellbook a FEW MORE TIMES.

Well, that's not entirely correct. Of course, if the opponent has a small army and spends a long time bashing your hydras, then maybe (although how could the opponent have a small army if your hero has 30 Spellpower?). Otherwise, in general, hydras die very quickly despite their HP, and realistically, if you plan to go into battle with only hydras, you will lose even with 30 spellpower, because you'll only manage to cast 1, maximum 2 times.
P.S. And I know about the strength of Warlocks in Chaos Magic.