The most useful monster.

The two most useful creatures in the game are the Chaos Scout and the Chaos Sorcerer. The Scout runs around the map, terrorizes the enemy, and tries to deplete the population of their cities, avoiding clashes with strong heroes. The Sorcerer, on the other hand, hunts down strong heroes in explored territories and eliminates them with magic and his army. The Scout runs around without an army. The Sorcerer has expert pathfinding, and the Scout has stealth. If you level up the heroes correctly and wage war skillfully, the effect will be amazing.
P.S. Heroes are the most powerful creatures in the fourth game.

1) No one will let them run around. To level up both (and the Sorcerer will definitely need to take Warfare), you need a lot of experience and time. The opponents won't be sleeping. For every Scout, there will be a fast hero who will automatically have the Scouting skill leveled up.
And the Sorcerer won't survive against a strong army; there are many ways to defend against his spells, and each race has them.

In general, I'm tired of the joke that the most useful creature is a hero. You might as well call the teacher the most terrible beast on earth:D But he/she is still a person.

On the topic, I'll say that all the casters in the game are useful units, i.e., mages (perhaps the least of all), ogres, djinn, vel, and Sorceresses. In general, fairies and imps are also very useful at the beginning of the game for speeding up heroes (but not for breaking through slow creatures with a single fairy).

P.S. No more words about heroes here