Like a couple others here, my rarest is also Mew* (Dragon Frontiers). Got it from a blister pack on Christmas one year. Out of all the gold star cards I could've gotten, I'm glad that was the one because I had already started up a Mew collection. Very cool to see that it's gone up in value, but I'll be hanging onto it.
Shining Mew (CoroCoro promo) is probably my next rarest. They're all over eBay, but most of the ones under $70 are damaged. I ended up getting one that appears flawless to the naked eye for only $30.
After that would be my English Neo Destiny Shining Pokémon cards: Raichu, Mewtwo, and Kabutops (unlimited edition). I pulled Raichu myself, got Mewtwo in a trade, and was given Kabutops.
And then there's Ooyama's Pikachu and my factory-sealed Dance! Neo Imakuni? card.