The strongest castle in Heroes of Might and Magic IV.

So, the conclusion is: there is no strong Faction, therefore there is no weak one either. They are all good in their own way. It all depends on the location and the map. Because some Castles (like Dungeon) need resources to build units, while others don't need much (Necropolis). In general, each location has its own surprises.

P.S.: If there was the strongest Faction, then what would be the point of balance?