I liked the archer upgrade for the Necromancer faction. Of course, it's not as good as the Lich upgrade in Heroes of Might and Magic III (when you get the Lich Robe), but it's still decent, especially with the Clone spell. I also liked Hellfire – it helps to alleviate the unpleasant impression of the Inferno faction. Arcane Wave is also a good skill for the Dungeon faction, but I never used Dark Ritual. I haven't played the expansions yet, so I don't know about the gnome abilities.