The most useful monster.

Those who say that the most useful monster is the one you have are right. :) And often you have to build based on available resources. Although, I'll go to great lengths to get Genies or Water Elementals into my army, no matter which faction I'm playing. Specifically for them, I'll develop Diplomacy or Charm for a secondary hero, and if I happen to find their dwellings, I'm incredibly happy! However, magic units alone won't bring victory. They are all just breakfast for the Chaos Demon. So, you need strong support units.

You can choose the most useful combination of units, but there probably isn't a single unit that is useful on its own.