The most useful monster.

I disagree; Heroes isn't 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, and others have slightly worse stats. And all of this is balanced by growth. It's wrong to talk about who is the "strongest" by pitting everyone against each other one-on-one.

Here, I agree, but not entirely. Heroes 4, compared to the other games, offers the possibility of a fair one-on-one fight, and that's what makes this game unique. Regarding the balance of power, I agree 99%, the balance here is excellent, except for minor shortcomings in the official expansions.

Then the strongest monster is the Black Dragon, in terms of stats and everything else, but if you pit two Phoenixes against one Black Dragon (an equivalent number), the Phoenix will easily defeat the Black Dragon...

The strongest of all creatures (specifically, level 4) doesn't exist.

Let's take the Dragon King (or Black Dragon). It will easily defeat one Sorceress in a one-on-one fight. But in a one-on-one fight with the Dragon King (or Black Dragon), it can lose to a Sea Serpent (approximately 1 out of 4 times). And the Sea Serpent will not defeat the Sorceress in a one-on-one fight.

Conclusion: There is no single strongest creature overall. This is one of the many factors that make Heroes 4 a unique game.