The strongest castle in Heroes of Might and Magic IV.

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

But that's a mistake; you shouldn't ignore the other factions.
It's immediately clear that the assessment will not be objective.
Any problem is best solved by considering all sides, and the situation (scope of the problem) needs to be specified, rather than just the number of options being considered.

P.S. That's when you get situations like with the "grostmasters" and their use of childish language.;)