The strongest castle in Heroes of Might and Magic IV.

So, we need to determine which faction is stronger in Heroes 4: Haven, Sylvan, or Necropolis. I understand that we are not considering the other factions.

You're misunderstanding... there are heated debates here. Everyone is trying to prove their point. All factions are involved. But Necropolis is losing in the discussions because its defenders can't say anything other than "Necropolis is the strongest and will crush everyone!".
But Sylvan is lucky... it has a very worthy defender in OptiMAL (respect!).

Who are Haven's ranged units? Hobbit and Titan? There are only 2 of them. Sylvan has 3: Archer, Ballista, and Monk.

OptiMAL, you're hurting my feelings :( Halfling, Mage, Djinn, and Titan - that's 4, compared to Sylvan's 3.