Arcane is the coolest in my opinion. It's a school of magic that's full of party tricks, that's pretty darn cool.
Walk on water, teleport to a party, magic up some wine, read someone's future and levitate some stuff while projecting films onto the wall with your mind and translating books with magic!
Followed by Necromancy. If I chose Necromancy however I would go straight for Dark Overlord of the World however. I mean, Necromancy doesn't have much use outside of world domination really. Infect people with magical plagues, raise the dead to fight under you, plague more people, raise more into your army, throw in some extra black magics and you've got the world under your thumb. Plus Necromancy puts you in complete control of your power, unlike Demonic magic which might be stronger but relies on the demons you rely on not turning on you. Which they will. Because they're freaking demons...