It would be...
  1. Apparation
    The ability to travel and see my friends and family in the blink of an eye would be the most fantastic luxury.
  2. I would really like to magically be able to understand a foreign language anytime I hear someone speak it. And then instantly be able to speak it myself.
    Suggested by @LizDawson
  3. Flying, in spite of having heard this years ago:
    Suggested by @biz
  4. Mind control
    Suggested by @amyfperry