The strongest castle in Heroes of Might and Magic IV.

Haven (if there's a hypnosis or berserk spell, and a cloak that removes immunity to magic, then it's game over for everyone) =)
Sylvan
Necropolis
Dungeon, Life
Barbarian because they struggle a lot against magic =)

But the strength of the Barbarians lies in their numbers, and they have always had about one and a half times more units than the other factions.
Necropolis is the best - a Bone Dragon with vampirism - that's real power.:D