The most useful monster.

I disagree. Heroes is not a game where everyone fights one-on-one; it's a "team" game, and the ability to manage a "hero + army" or "unit + unit" team is important. That's why some units have slightly better stats, while others have slightly worse stats. And all of this is balanced by growth. To talk about who is the "strongest," even if it's a powerful unit, and pit everyone against each other one-on-one is incorrect. In that case, the strongest monster would be the Black Dragon, based on stats and other factors, but if you pit two Phoenixes against one Black Dragon (with an equivalent number), the Phoenixes will easily defeat the Black Dragon.