Whatever I've got on-hand, nearby or is most likely to stop the pain the fastest, and last the longest.
If I'm buying it, I'll just choose one at random, depending on the pain. Some 'painkillers' are more adept at stopping certain kinds of pain than others. Acetaminophen (Tylenol, Motrin) is best for pain and fevers, Ibuprofen (Advil) is better for plain pain. Aspirin works nicely too for plain pain, but I'd only use it if it was handy and no other was nearby.