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.
My Favorite urban legend is about kids prank calling people but they end up calling a serial killer that goes after them. not too sure what urban legend that is exactly but i heard Dead Tone is a movie coming out based on it, however, i think its being banned for being too graphic.
I don't know if there is a name for this legend, it's about some guy/girl who's just about sitting down in a seat in a movie theatre and suddenly something stings him in the ass. under the seat is a message "A needle with my blood has stung you, you have now HIV!" A lot of my friends heard of that and believed that somewhere is maniac spreading HIV...