Rayman Origins all the way
This game's art direction is unbelievable. Really feels like an actual cartoon moving.
Honorable mention goes to...
One of my favorite things about the GBA Wario game (which are my favorite games of all time, just so you know) is in its art. Wario Land 4, Warioware Inc and Warioware Twisted have this crude, gross offensive and broderline comedic art style that really distinguishes it from the rest. There's nothing truly like them, and they pretty much reflect Wario as a character. Wario land 4 in particular has some really amazing background design as well, which is par on course for the team behind Super Metroid.
Rhythm Heaven doesn't even look like a game. If you play it perfectly in front of someone, they'll wonder if you're playing a game at all. Rhythm Tengoku clearly has some Warioware sprinkled in it, and each subsequent game is cleaner than the one before it, it's really nice looking either way.
I can't leave Wario Land Shake It out of this. It actually feels more like a cartoon than Rayman Origins, and the animation are fantastic. This is all hand drawn people. I give it to Origins purely because the game has a distinct style, but Shake It is nothing to scoff at.
The sprites are hit or miss, but the background design is amazing. Oh and by the way,
this music plays for this level. DKC2 does atmosphere like no other, thanks to its art design AND sound design
I'm not particularly sure what I exactly like about Ouendan/EBA. Maybe it's the Manga/Comic book art with its own style and the overyly awesome exaggerations. It's actually mostly the character design, though.
I actually ADORE what Next Level Games managed with Punch Out Wii. Whil Origins/Shake It are borderline cartoon, this is what cartoon looks like in 3D. The very distinctive look of each boxer combined with the animations make this a gem.
I gotta give it to Super Metroid/Metroid Fusion. It does isolation so right that it's still unmatched to this day. There's something very unsettling in its art, particularly Fusion. The environment design is just amazing.
One more, and I'm giving it to Tropical Freeze. I always loved Retro's art and environmental design, but I'm particularly fond of this game. Each level is just filled with details in the art and environments, that it makes sense it took the game 3 years to be developed with all the heart put into it. Absolutely can't wait.
...Too much? Had more in mind. lol