Mine is a mind-reader effect, which is really just a force and a fancy way of finding the card, but it never ceases to amaze. A riffle force (name?) where you have a break at your force card, you riffle down the pack and tell them to say stop and lift up at the break. I then immediately put the top half back and give them the deck to shuffle. Then I spread out the entire deck on the table and tell them to mentally send me the image of their card (if it's a girl, I'll ask her to hold my hand to "improve the connection"

I know some other effects and plenty of utility moves, but none are as simple, clean and effective as this one.