Ou of all the cards that have already existed, what is the most rare one, from the Gen I cards to Gen VI cards? And what makes it so rare?
Pretty crappy answer but it is a set of five cards which were made because a compition was made to redesign a few Pokemon. The winners got their card printed and a "Ilustrator Card" which certified they won it.
The Illustrator card is worth in the range of $10000 - $20000.