The ability to make all idiots you meet to instantly feel like they have been punched in the face.
I'd use it all the time.
You know, that fits perfectly under my illusion/experience ability, but mine is much more versatile.
My next one; the ability to control what people notice.
You go to a stadium for some popular sport, filled with people. You then light a person's hat on fire, and prevent them from noticing. The fire spreads to their hair, then their clothing a flesh. They don't notice. As the burn away on the spot, the person beside them also catches fire, and does not notice. The first person complains of being tired (as his muscle tissue is being burned away and the fire is taking oxygen), so he goes back to his car, lighting the inside on fire and finally killing him. By this time, the fire is spreading rapidly through the crowd as you watch. People being engulfed in flames and dieing, while no one notices. Eventually, the entire stands are a great wall of fire, while the center the players play on as though there's nothing wrong. Someone's flaming foamy hand falls into the stadium grass as he collapses, lighting it on fire too. When the flames have eaten everything within, you deactivate the ability. So a while later, when the clean-up crew comes along, they are horrified as they look about the ashen remains of so many people. All over the world headlines of "Instantaneous Combustion of the such-in-such Stadium!" are heard and seen. Decades later, scientists and random people still work to crack the mystery surround this stadium, it even turns into a tourist spot and a memorial is built.
Of course, I would never use the ability for something so horrible, I don't have anything (much) against sports people. I would probably do the opposite, making people notice things that require attention. And yet, I can't think of an example off the top of my head for this. Other than city-blight, and select scandals.