If you could cast one type of magic spell, which would you choose, and why?
One TYPE, or one SPELL?
One type... probably transportation spells. They're just so damn USEFUL. I'd be able to fly, teleport, transport large and small objects, etc., and clever usage also permits me to use them for offense and defense if needed.
One SPELL... well, can I go to ANY source for the spell? Because the sources I use would have a big effect on what spell I'd pick!