The most useful monster.

I'm for vampires; they are useful in all Heroes games.

It's hard to argue with that. Vampires are very strong, especially in Heroes 4 (the strongest creature of level 3). They are especially useful in a prolonged war (although they are also useful in a quick one). If you have about fifty vampires in your army, you have nothing to fear. But still, I would choose 15 Dark Champions rather than 50 vampires. They have regeneration and the ability to cast the "Terror" spell. All this makes them much more useful than vampires. Their usefulness lies in many factors that allow them to win in almost all situations. But unfortunately, a Dark Champions' Sanctuary cannot be built in a city. Otherwise, they would be ideal warriors and would share third place in strength with sea creatures.