This is a tough one, as I have several card arts that I absolutely adore. However, one that I love the most (at least, at time of writing) probably has to be the FA Mega Gardevoir from the Generations Radiant Collection. Firstly, I like it because... well... Gardevoir. Second, I love the coloring on the card: the black mixed with the dark red looks rather nice, especially with black sleeves. I remember back when the Brilliant Arrow Mega Gardevoir was a rather good card, and remember really wanting to build that deck despite having pretty much no money whatsoever. In fact, I kind of still want to build it...
Anyway, other than the Mega Gardevoir, I'd probably have to say the Delta Species Flygon from EX Holon Phantoms. I've owned one for a long while now (probably since I was about 8 or 9), and have always loved it.