Bastion-Not only for it's lush handpainted like graphics, but it's stunning soundtrack and narration, it's so hard not to be engrossed the entire time in it's beauty
Final Fantasy IX-Prettiest thing on the Ps1
The Elder Scrolls III-Morrowind: It might not be in HD, but I felt Morrowind's world was much prettier then say Cyrodill and Skryim, Cyrodill was very pretty, but rather generic, and Skryim is just a big grey mess imo
Mass Effect Series: sure some of the unmapped worlds were kinda uninspired, but almost everywhere else in the game was really good, the same goes for KOTOR and KOTOR II
Fable: For it's time Albion was a realllllly good looking.
Flow and Flower: Stunning
The Bit Trip Series: Maybe it's for the nostagia of old, but I think the Bit Trip series looks wonderful
The Metroid Prime Series: 0_0
Halo 2 and Reach: Both were really, really good. Reach was such an interesting looking planet.
Monster Hunter Freedom 2 and Monster Hunter Tri: Both World's were stunning, and both pushed the power of their systems to do so :)
Final Fantasy X: To this Day, the most (Imo) the most beautiful RPG world I've ever set out in
Dragon Quest VIII: My favorite use of Cell Shading to this day, though other recent titles do a really good job as well (Tales Series, Eternal Sonata, etc)
Metal Gear Solid III: I guess this one is just in the eye of the beholder, but man, did that game ever look so good to me, those Jungles were so engrossing for me, the animations on the animals, your camo's, it was a diffrent sort of beauty, but idk, it counts to me.