The weakest castle in Heroes of Might and Magic IV!

But that's only half the army!

Are these Djinn half the army? O_o The enemy will blow them away without even noticing...

and if we're going to delve into units, then not all units in other factions are "top-tier."

I don't know what's so great about "top-tier"... Well, attack/defense can be relatively good, and they have 400 HP, but you can't heal/resurrect/apply buffs to them. As a result, you always need to be focused in battles.
In my opinion, Phoenixes and Dark Champions are probably slightly better than the others; the former thanks to their huge speed, good HP, powerful attack, and regeneration, and the latter thanks to regeneration, high attack/defense modifiers, a charge attack, and "triple" terror.
The rest are roughly equal. This is specifically about battles with neutral creatures.

You could also take statistics from, say, 1000 tournament matches and see which faction wins/loses most often.

You can, of course, look at the history of tournaments or the Heroes leagues, but that's not a very good indicator. Different factions dominate on different maps, and most outcomes depend on the player's skill.