While I love destruction magic, there is something special about Illusion especially when you get to master level spells coupled with quiet casting. I remember casting the master frenzy in the Falmer caves under the lighthouse between Dawnstar and Winterhold once. It cleared 3 rooms with the single casting because the tunnels were twisty and I guess the spell goes straight through walls. All that was left was usually a Charus reaper which are pretty nasty on their own, but at least I didn't have to fight multiples of them at a time.