God this is hard, there are just so many gorgeous ones.

Ultimately though, I'm gonna have to go with Captain Toad Treasure Tracker. It's been a cliche for years that a video game "looks like CGI", but Treasure Tracker truly does. It's polished to a celestial sheen, immensely cute and charming, vibrantly colourful, it never drops or tears a single frame, and everything just fits together into a near flawless cohesive whole.

Damn right. It's insane how well they hold up after so many years and so much hardware improvement. I think their secret weapon is their use of geometry. Where so many games are full of square corridors and rectangular buildings, the Prime games are overflowing with organic, handcrafted geometry, from cracked rock to twisting vegetation to alien architecture whose unique culture is reflected in distinct polygonal styles.

Doesn't hurt that they run at a slick 60fps either.

mario kart 8, botw doesnt look nearly as good

umm WindWaker HD, MK8, Pikmin 3

I can see that. Mario Kart 8 is beautifully detailed and most of the low resolution textures are unnoticeable, something that doesn't happen with Breath of the Wild, which happens to be more inconsistent. 

On another note, I'd say I'm in love with Captain Toad: TT. 

Honourable mentions:

Yoshi's Wooly World

Kirby's Epic Yarn

Kirby and the Rainbow Curse

Super Mario Galaxy

Metroid Prime 2: Echoes

Metroid Prime 3: Corruption

Can't really pin it one one game, there's a lot to consider. Obviously, newer games look technically better than older games. However, what if you take your personal esthetic preferences or art design into account? Or how a game looked relative to it's time? Or look at it by it's bare effects and technologies?

I'd say, taking those three parameters, my winners would be;
Esthetically: Yoshi's Island (SNES)
Time-frame: Super Mario Galaxy (Wii)
Technically: Captain Toad Treasure Tracker (WiiU)

The last one is hard, does Captain Toad really look better than Mario 3D World or Mario Kart 8? I can't believe I'm not mentioning Breath of the Wild, but from what we've seen it still has a couple imperfections. Can't wait to see the game's graphical quality on NX though.

If you're going to make a dozen+ threads a day, could you change your avatar from some dementor eating the brain of a some poor soul trapped in hell? :D

Otherwise, I struggle to think of a Nintendo game that didn't have a beautiful artstyle. They may have been stuck working with inferior hardware in the Wii and Wii U, but they know how to get the most out of that hardware.

Skyward Sword. I absolutely loved the impressionistic paintbrush art style there. If that game had an hd remake, it would be breathtaking (Breath of the Wild looks amazing, and I know they say it was a continuation of SS, but it clearly looks more anime than impressionistic).

Edit* I wouldn't be surprised if someone says the Zelda U tech demo lol

you can't really compare Nintendo games.

They all have their own spin on visuals so its hard to pick one when each game does something better than another.

I think Kirby and the rainbow paintbrush looks beautiful with that clay artstyle, Captain Toad and Yoshi woolly world stand out but Mario Kart 8 takes the crown for me