Rare Pokemon cards include the first edition holographic Charizard card, the pre-release holographic Raichu card and the Illustrator Pikachu card. The Charizard and Raichu cards were both released in sets, while the Illustrator Pikachu was a promotional card.
The first edition holographic Charizard card came in the first set of Pokemon cards. A shadowless version of the card, which doesn't have the standard shadowed bevel in the main image, is the rarer and more valuable version. Shadowless versions of the card with a high condition grade are worth over $1,000, as of 2015.
The pre-release Raichu card has the word "Prerelease" stamped in the bottom right corner of the main image. Only eight of the Raichu cards have this stamp, making it worth around $10,000.
CoroCoro, a magazine which once sold Pokemon, created the Illustrator Pikachu cards. It gave the cards, which have an image of Pikachu holding a paint brush, to winners of a Pokemon card illustration contest. Only six of these cards exist. Ungraded Illustrator Pikachu cards sell for $20,000, and those with high grade are worth much more.
Most Pokemon cards have a rarity icon on the bottom right corner. This rarity icon indicates how often the card appears in booster packs.