My favorite card is the last card in my collection, a card that should have never existed, featuring a player most people forgot existed and may or may not have a lot of value or none:
1999-00 Upper Deck #164 Pavel Brendl UD Exclusives. The card came out a year before it should have much like the Jason Spezza Young Guns that came out in 2003. I've been told that anywhere between 1-4 of these exist. It also never got the serial number for some reason.
If only it were 2002... I think I may get it graded some day.