Probably this one.
It's a promotional card released only in Japanese, and was awarded to those with a winning entry in the Pokemon Card Game Illustration Contest.
Currently, there are only 6 known copies in the world, and only 4 copies have ever been sold. Apparently, it has sold for over $20,000 in the past before. And remember, it would be equivalent to alot more today (inflation, etc.)
Also if you take a look at this site, it shows someone a year ago trying to sell one for $100,000. He got a bid of $47,000 for it apparently, however I'm not sure how much he sold it for in the end.