I think I'm going to buck the general trend and say terramancy (earth magic, in other words). Don't get me wrong, fire and lightning and necromancy and such are great, but...A fire can raze a town that requires years to rebuild, but the earth can swallow a city and make it as if it were never there. It's hard to necromantically raise undead legions when all that's left is bone dust, and a sink hole or rock fall is a damn good answer to "oh no, zombies!" I can conjure volcanoes, just by going deep enough. If I am standing on anything, I have ammunition. My citadel is built to my specifications, because I made it that way, and good luck getting to it if I don't want you there. The best part? People will almost always underestimate you because, when you say "earth," most people think "dirt."