* Millennia of wisdom and practical leadership experience
* If you can magic, she can teach you how to magic harder than loving Dumbledore
* Probably nicer than your own grandmother
* Hardcore enough to adorn her castle with pictures of her greatest failure so she would be reminded of it every day for a thousand fucking years
Was going to pick Goku/Gohan or perhaps even Master Roshi, since even achieving Krillian or Yamcha level of Ki manipulation would make me the most powerful being on earth, but damn you make a good point about Celestia. Though I think magic is species-based in Equestria, and no human has been shown to be able to use it without the initial aid of an external talisman (Twilight's Crown, the Siren's Crystals) so per the rules a human being is unable to learn it just from mentorship.
The problem with this answer to me is Tony is a dick to everyone he sees as beneath him. The closest we see to him mentoring someone in the movies is the kid in I3, who he's a giant ass to.