The best faction is Necropolis!!!! Necropolis rules everywhere!!!! (In all games, and here too)
Yes, on large maps, Haven rules; I think it was designed for that. Dark magic, as already mentioned, should be feared by all large armies, but unlike others, Haven suffers the least because the hero can cast Cleanse and the monks can do it too. In my opinion, this is the only race where the full potential of the race is revealed precisely at the end of the game, although, no, there are also Necropolis. I don't understand why Haven is considered weak; for me, a player with 4 heroes is the strongest race...