That'd be either Yukari or Rumia. Excluding stuff like "infinite wishes" (i.e "now you swear to serve me forever"):
Yukari:
Extremely ludicrous theft
Take me to various places of the world that i wouldn't be able to see otherwise, including Gensokyo and the Moon (if it's full Moon time), while protecting me (you never know with those youkai deals!)
Use some (permanent) crazy shikigami equation to make me faster, stronger, smarter, and so on.
Allow me to pet Ran and Chen :3
Give me her fan and umbrella
Make me fly on her/Ran's/Chen's back.
Tell me the truth about her powers.
Rumia:
Make me fly
Destroy some stuff for the heck of it
Eat my mother's boyfriend
What if it's actually a trap and everything you ask of them comes back at you in some way or another?