My favorite has always been "Time Vortex."
Not because of what it does - but because of what it's represented. It's a "Rabbit Promo" and for the first few years of its existance the only way you could get it was through a Rabbit. It wasn't a card associated with any convention (like the Dragon*Con or Origins promos) - you had to know a Rabbit.
Way way back when I first became a Rabbit, I got a kit for signing up and doing demos - and it included some Time Vortexes and my first Carrot Box. I've always been proud to give them out - and proud of the Rabbit program, what it grew into, and what it's become.