Two of my favorite cards are Entropic Specter and Ravenous Baboons. The specter is just great fun in multiplayers, filling up someone's hand while making somebody else discard and the baboons were a way to stick it 'the rich guy' who had a deck filled with non-basic land while you were playing all basic all the time.
But I'll nominate Mogg Assassin.
The randomness is weird, but the politics are overwhelming. Definitely a card for multiplayer.
I nominate Spellbook. Exodus has some cool power rares that would also be contenders for me. But Spellbook was the first (with the non-notable exception of Anvil of Bogardan) and best at doing something many players (myself included) found to be particularly special. There were all sorts of spells that could load you up on cards, but for those that want to have some fun and keep those cards around for a while, Spellbook has consistently remained one of the best options.
I have to go with Coat of Arms. It was a really good idea, and a whole new concept, all rolled up into a great artifact. It gave other tribal decks a fighting chance against our last winner, the Slivers!
Pandemonium saw play a few blocks later in a nasty combo, but in its heart it's a casual card. It encourages big creatures and lots of back and forth, and before Saproling Burst helped out fun combos like Sneak Attack and the like.
OK, I think it was a unanimous decision. In fact two unanimous decisions...
Mogg Assassin got everyone's vote in the end, but I think all three committee members were disappointed by the quality of nominations this time (perhaps to do with the fact we didnt get a list of cards posted?). I would have expected to see cards like Jackalope Herd, Erratic Portal, Equilibrium... and the selections we had I think we felt were either too good, or too random, but nothing that was just right (this is sounding like Goldilocks now) apart from the Assassin.