Derren Brown's Smoke is a killer effect. I don't smoke so I usually reveal the card in another way, but it is without a doubt the best effect I present.
Also, I think the title of the thread could be changed by a moderator. Rather than best trick, it could be best effect presented. It does not matter what trick you do, but your presentation of the trick. Some tricks can stand out by themselves with zero presentation and amaze your audience to a level, but any good patter and presentation can perfect the effect.
David Blaine has such good presentation with his Monte's, that even though they are not complicated tricks with gimmicks and gaffs, audiences are still left baffled.
Other good reactions for me though are Witness, New Era Moving Pips, Distortion, and This That and The Other. This, That, and The Other is simple, but it can really work a crowd.