The first Meowth card because it was one of the first cards that I got, and being a Meowth fan, I will always treasure it. I also have a soft spot for the cards in the first Neo set like Togepi, Wooper, and Ledyba since I have fond memories of attaining them.
That's a tough question... We always like different cards for different reasons (e.g., for their use in decks, for our memories of how we obtained them, for the design, etc.). One that stands out for me is the holofoil Gengar from the Fossil set. Gengar was always my favourite Pok'emon, and I got a Fossil booster pack from a magazine before the set had been officially released in Australia. The rare in that booster pack was the holofoil Gengar, which was the card I wanted the most at the time, so that's probably my favourite. But Blaine's Arcanine (Gym Challenge set) and Shining Gyarados (Neo Revelation set) are close behind.