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

I think that out of the ones listed, liches (archliches) are the best:
1) archer
2) mage
3) excellent melee fighter
4) necromancers (raising the dead and other advantages)

And if the enemy has them, the most dangerous are Adepts (renegades): they cast blindness, shoot well, and have a lot of health. Although priests are also not very weak.
Hydras have one major drawback: by the time they reach the enemy, there's usually nothing left (others manage to defeat them), so they are good in a "fight to the death," but it's not possible to "quickly deal with" enemies using them, unlike liches.