There are way too many to choose from but my personal favorite is the Bloody Mary legend. I have scared the shit out of so many people that do the mirror thing.
Gators in the sewers of New York. I have heard other variations of Bloody Mary, but it's still the same trick with the mirror in the dark. "Biggie Smalls, Biggie Smalls..........Biggie Smalls."
alligators in the sewers, earwigs climbing inside your ear or if you drop a coin from the top of the empire state building, and if the coin hits a pedestrian (who's at ground level) on the head, the coin will fracture the skull and kill the pedestrian.
i like the first one because i like the idea of famously wild animals living in an urban environment. the second one was on a classic episode of the night gallery that use to scare me as a kid. and the last one was tested on Mythbusters, the myth was busted but the coin travelling at those high speeds hitting you on the head would hurt well more than i thought.
I always enjoyed the one where if you make a bad face, and someone slaps you on the back, your face will stay in that position. Though I doubt parents still tell kids that one, if you ever see the movie One Crazy Summer, it happens right near the beginning of the movie.