The most powerful response I've ever gotten to a card trick is "The Dark Card" by the Camirand Academy.
The basic effect is that the spectator signs a blue-backed card which is revealed at the climax to be red-backed.
The difference with The Dark Card is that the spectator handles the signed card freely and can see both sides. It's a blue-backed card when they sign it.
I have used two routines with this. In the first, the card is signed, then revealed in a repeat sandwich, followed by a repeat card to envelope. Finally the back change is revealed as the climax.
I've also used this in an Ambitious Card routine in which the ambitious card is signed by several people around the table during the routine.
My sister's reaction says it all. "Those other tricks were impossible, but this... this is just unimaginable."
If your dealer doesn't carry it you can find the Dark Card at Camirandmagic.com
. Best ten dollars I ever spent.