I'd have to say the one about Bloody Mary. If you splash water on the mirror, turn around and say "Blood Mary" 3 times, when you turn around, she'll be in the mirror.
Sorry I have to tell my little story about this.
When I was 15 we all went on a camping trip with my air cadets, now its a tradition the first night for the older cedets to scare the hell out of the new recruits.
So this first night there was a major thunderstorm and it was so beautifull all pink skys with lightning everywhere but we where told we had to stay in the changing room cubicles for the night because the tents where not safe.
Being typical teens we decided not to break tradition and scare the hell out of the younger ones so we told them about bloody mary, we ran through the whole story building up suspense and it was great that the power had all gone out and lightning everywhere so we only had torches.
The plan was to then take them one by one into the loos and get them to stare into the mirror and then just scare them, others outside hearing the scream would add to their fear (hay I never said we where pleasent teens).
So I take the first one into the loos and shes staring in the mirror, im about to pounce behind her and scare her when all of a sudden this massive bolt of lighting hit the ground just outside and lit the whole field, building the lot up.
She screams spins around clobbers me one by accident, I ran for the door just as someone else screams and runs in and knocks me out cold with the door.
I cam round a few seconds later to find everyone running round like headless chicken screaming and crying so I went to open the main door just as anouther bolt of lightning shot across the sky and lit up the face of our officer standing in the doorway coming to see what all the screaming was about, I pretty much craped myself and we all swore that the tradition of scaring younger cadets ended that night