The Proposal Rejection Prank is a perennial favorite. Back in 2005 I posted about how a couple had developed a routine they were performing at basketball game halftime shows, in which the guy would propose to the girl in front of the entire crowd. But instead of saying yes, the girl would break into tears and run away. It never failed to get a reaction from the crowds.
The PrankvsPrank duo recently performed the identical stunt at various locations outdoors. Their resulting youtube video currently has over 4 million views.