It would be the ability to make my phone ring at-will. I'd have to be careful to not overuse it because I don't want to be rude, but think of how useful it could be!
That annoying coworker has cornered you for the 8th time this week (and it's only Tuesday) to tell you about how the government is putting microchips in our peanut butter? "Oh, hey, I'm so sorry, I've got to take this!"
Shop staff keep following you around and trying to sell you tea services that you really don't want or need and couldn't afford anyway and won't take "I'm just browsing, thank you" for an answer? "This is super important, I'm sorry!"
It would probably be the most useful non-useful power ever!
I have no idea what I'd want for a real, useful power, though. Obviously, it wouldn't be this; I'd want something that could benefit people other than just myself and that would be useful for years to come.
What about you? What would your stupid power be? How about your useful power?