Which faction poses the greatest challenge for the Dwarves?

I think the Necropolis has a good chance, especially if there are multiple castles. A Dwarf might try a rush with Ingvar early on. But then the Necropolis will have too much meat.

I'm considering a game with an equal number of castles. And I don't think Ingvar's rush is that scary. The main thing is to get the Fire Warriors summon spell as quickly as possible; in any case, he's unlikely to capture the castle. You can also quickly get several high-level creatures through raising on a rush, while the Dwarf will only have warriors and pikemen. In the late game, the Dwarf will either have Armageddon with Pyrokinesis or Eternal Light. Plus runes. Invisibility and Battle Fury. The Necropolis won't have that much meat (if the number of castles is equal).