Don't know if it would necessarily be deemed the "best" comeback, but typically when somebody does something d u m b or is being a p r i c k I tell them something along the lines of "The people who tolerate you are the real heroes of the day" or "Do your parents know that you're living proof two wrongs don't make a right?"
It all really depends on what the person did or said. Otherwise, a middle finger is quite the reliable comeback.