Light or Dark magic.

Darkness against necs — original. Personally, I always level Light against necs (some darkness spells don't work on them).

Original, but on some maps it makes sense. For example, there are Dragon Utopia (read: free level 5 spells), and if you weren same given Anti-Magic in your home guild (or even left without Dispel Magic), then Vampirism will save you from Berserk and other nasties. Leveling Darkness is even more convenient, and there are useful perks there. And against living races, a Knight's Vampirism rocks: level it up until ready and cast it on alt. griffins, and the birds will most likely sing "My Immortal," plus you can cast it on Zealots of Faith and Isabelle's knights. And this miracle costs only 10 mana. Perhaps only necs and TE can use Vampirism as effectively.

But lately I still take both, dealing with the lack of spellpower using Twilight and a Fallen Knight....

Get by with artifacts, education, and a Fallen Knight, because 2 magics is almost suicide for a Knight.