All races were balanced as much as possible. So, the stronger race is the one played by the more skilled player.

I used to think that Dungeon was the strongest because of its powerful troops and magic. But its strength is compensated by its relatively low survivability (except for hydras, but they, on the other hand, deal weak damage), and Chaos is only good at the beginning and middle of the game, and only with high spellcasting skills. Now I think that with skillful play, the strongest race will be the Academy. They have decent troops, an excellent racial ability, and simply magnificent spellcasters. If you develop the spellcaster well, they can effectively support their troops with spells, and if you give them minor artifacts, the level of the creatures will effectively increase by one. Summoning a phoenix at the right time can decide the outcome of the battle. Necropolis is a little weaker in this regard, and humans and demons can only win due to their huge armies. Dwarves and orcs are interesting in their own way; a truly skilled player can defeat even a spellcaster with them. But this also depends on the situation.