Anything from Leader Games and illustrated by Kyle Ferrin (Root, Oath, Fort...) feels like cheating but still, I absolutely adore their art style
Everdell, Photosynthesis, Parks, look nice as well
I also bought an updated edition of Costa Ruana just because of the illustrations: https://www.pamwishbow.com/client-work/project-four-yjynj-pafyx
For me, the game with the most beautiful art has to be Ashes: Rise of the Phoenixborn.
My favorite art style, though, would have to be Middara (though the female characters could use some more practical outfits). It is just so over the top and vibrant.
Another art style that I enjoy seeing is Dungeon Degenerates. The gross, ugly look fits the atmosphere of the game setting.