Twilight and her ally, Spike. The complete lack of explanation for unicorn magic made me go for a less canonical expression of Twi's magic. She has teleportation, shielding, and a magic blast as spell all the time. She has those spell's prerequisite, as well as prerequisites from many other colleges. Finally, she has three 4-point modular abilities slots. She can fill these slots with any spell she reads in a book and she has the prerequisites for. It takes her an hour per slot to prepare a spell this way.