The rarest card of all is the 1996 World Champion. This card was given out to the winner of Worlds in 1996, and all other copies of it, plus the printing plates, were ceremonially destroyed. The winner of that tournament was Australia's Tom Chanpheng, who, at last word, still has the card at his home in a screwdown plastic case.
Chanpheng won the title playing a White Weenie deck:
World Champion 1996
The funny thing about Chanpheng's deck was that it contained a Sleight of Mind with no way to cast it. He misregistered his deck, forgetting the Adarkar Wastes. DCI rules forced him to fill out his empty slots with basic Plains, and so the Sleight of Mind was "stranded." It didn't matter, however, as he went on to win the Championship anyway.