The Greatest and Most Powerful pony in all of Equestria (in other words, Trixie trixied Twilight Sparkle into casting a spell) has summoned you and transformed you into a unicorn.
Unfortunately, something went wrong during the conversion process, and your horn is only capable of casting a single spell. Twilight Sparkle will teach you any one you want, be it teleportation, Summon: gossamer & morning dew wings, telekenisis, or even "get me the hell out of this pastel colored crazy town" (complete reversal). It's up to you.
What do you choose?