WOW, good question. I would have to say it's a 3-way tie: (1) Signed Card to Ceiling; (2) One Cup Routine with three lemon final load production; (3) A Magic Square Routine (I get the "thought of" number using a p _ _ k wallet.)
I think most of us do try to make our tricks and routines as memorable as possible but for some reason, specific tricks seem to outshine the others in the mind of the spectators.
In reference to one of my previous posts about tricks you hate doing that your audience loves, I've heard stories of great magicians (so this probably isn't an issue of presentation) who no matter what they do, the one trick their audience seems to remember is their spongeball routine.