I liked the archer buff for Necromancers. Of course, it's not as good as the Lich buff in Heroes of Might and Magic III (when you get the Lich's Cloak), but it's still decent, especially with the Clone spell.
I also liked Hellfire – it helps to alleviate the unpleasant impression of Inferno somewhat.
Arcane Wave is also not bad for Dungeon, but I didn't use Dark Ritual at all.

I haven't played the add-ons yet and don't know about the abilities of the Dwarves.