If you mean Ultra Sun (just in case), I suggest to use it on Mewtwo. Mewtwo has Recover, and was a bit annoying to deal with for me. Of course, you could use it on any other Legendary you want (or Tapu Lele), but Mewtwo is considerably harder to catch if you don't have Heal Block and the Master Ball matches Mewtwo's color scheme.
If you mean Sun (because I read the question wrong), Give it to a Pokemon and offer on the GTS
, People will desire it more; or use it on Tapu Lele, I like how it matches the color scheme of the Pokemon, it will really complement it. Otherwise, use it on another Pokemon that matches it's color scheme. It looks well, but be aware that you can't pass down Master Balls via breeding.