My favorite Seviper card is Psychic-Type, so I’ll go with that one:
As for some other of my favorite Psychic cards (and if Seviper wasn’t my favorite Pokémon, I’d likely would have gone with Natu):
Love the Natu and Bouffalant pair as a short story, but even just either artwork loose is very good.
The Cresselia colors and background are full of details, which I’m always a fan off.
The Double Crisis set is my favorite set of all, because almost all Pokémon cards in the set are very expressive and tell short stories of the Team Magma vs Team Aqua battles.
Gardevoir has a lot of amazing artworks, of which these two are just the tip of the ice berg. The first one where Gardevoir stands in a field of flowers has great use of shadows and lighting, even though it’s subtle enough to barely be noticeable on first glance. Seeing them side-by-side like that, it’s just a coincidence they have a similar expression with closed eyes. 
They’re mostly my favorite among the Gardevoir cards based on the backgrounds.
The illustration winner of the Gourgeist did a great job giving off a creepy/spooky vibe in this artwork that contains 33 different Ghost Pokémon in a single artwork.
Almost all Unown cards are pretty solid imo, although none really stand out big on top of the others. The main reason I like this one the most is because the red flowers reminds me a lot of the scenes in the third Pokémon movie, and it’s also one of the few very vibrant and colorful Unown cards, where most are on the darker side.
And as a FA Supporter/CHR collector, this story-telling trio of Ralts, Kirlia, and Gardevoir growing up alongside their family is very touching. I hope we’ll see a lot more of these kind of artwork stories among multiple (preferably full art) cards.
Greetz,
Quuador