Time stop. Preferably either the 2nd ed version or the Master of Magic version (latter version stops the entire fucking world, except you and your armies and anything within a mile or so of either. And can last several years). Former version would basically turn me into a super if I can cast it whenever I want, since I can pause events at will and adjust as needed. Latter version would basically turn me into the general from hell because I can trap the bulk of any enemy forces and only hit key spots (sadly, a less useful ability in today's society, but it could also be exploited economically).
Or Polymorph. 'Cause that's just fun.