Only judging games I've played, and of course based on my opinion of how pretty the art style is. If we're trying to be objective about the best graphics, there shouldn't be much of a debate in the first place, but let's be real, most games nowadays are very pretty.
1. Hollow Knight
2. The Legend of Zelda: Breath of the Wild
...and that's it, those are the two I feel like pointing at right now. As I said, most games this gen are very pretty, I could throw shout-outs to the likes of Super Mario Odyssey, Captain Toad, Splatoon 2, Crash N. Sane Trilogy or even DKC: Tropical Freeze, but there's just too many. Metroid: Samus Returns was particularly impressive for its hardware, but it won't look good on anything other than the 3DS screen. The best-looking game I have played was probably God of War, though my time with that was brief enough that I don't feel like talking about it.