Dang, that's a tough one. My favorite card arts are almost all from the XY series, whether they be normal cards, full arts, or what have you. Here's my top five:
#5 Skyla (Full Art) - BREAKPoint
-Yeah, it looks strange (guys know exactly what I mean, don't even try to front), but it's definitely better an the Boundaries Crossed one imo, and the color scheme is really well done.
#4 Gardevoir (Shiny Secret Rare) - Dark Explorers
#3 Hoopa (Full Art) - Ancient Origins
-I love rainbows. The background to is glossy card is rainbows. How can one dislike a rainbow?
#2 Any of these:
Every card art for the LEGEND Archetype.
Those looked unspeakably amazing in design.
Have always loved these cards, the art was simply phenomenal on them, and it's hard to accept it when someone can't acknowledge the artists sheer talent with these. I remember when I was much younger, I had completed one of these things, I think the Entei and Suicune one, and I was blown away by the artwork.
#1 Glaceon (Full Art) - Fates Collide
-Yeah, I love my full art cards, by far the prettiest set of cards there are overall since LEGEND archetypes. Glaceon, my 2nd favorite Pokémon, in a highly playable,
beautiful card (my favorite hasn't been released into the TCG yet, since the game it's in doesn't release until November 18th...).