The strongest castle in Heroes of Might and Magic IV.

Hmm... I wouldn't play Haven on S or M maps. Genies are cool, but they are quite expensive, and in small numbers, they are even worse than Imps. Besides, they aren't that effective against Necropolis.
But in general, everything depends on the game version and the map. If you play on a "beefy" large map in Heroes of Might and Magic, Necropolis can gather a lot of Vampires, as the hero can raise them starting from level 8 (this was fixed in the expansion - level 18). Chaos is obviously weak on large maps - Chaos Knights don't get stronger with magic, and magic is generally useless against enemy heroes.