Gen 6. I can resoec that 3d won't be everybody's cup of tea, but the Pokemon models were quite beautiful in my eyes, looked solid from all angles and had lively battle animations, including the scenes for mega evolution. Every Pokemon also had a lot of expressivity and charm when you got up close and personal with them for interactions in Poke Amie. I spent a lot of time there, just petting my mons, feeding them, admiring their beauty.
XY is a short game that doesn't frequently use cut scenes, but the ones they had did make use of the visuals in a way that got my attention like when the cover legendary appears it is breathtaking, you get the sense of the power and otherworldly qualitiy of that Pokemon as it breaks free, and think the story of AZ's floette was also told very sensitively, and think the delicate animation that looks like faded illustrations on pages from a book lost with time, is nice.
Kalos Pokemon also mostly looked excellent in terms of design. There were some real beauties introduced in gen 6 like Xerneas and Auroras, Noivern, Talonflame, Pyroar, Vivillon, sleek Pokemon with attractive and alluring color schemes, graceful shapes that could really look majestic. It also created some cool-looking mega evolutions for existing Pokemon in both XY and Oras like for the Ralts line, the Kanto & Hoenn starters + the weather trio and more.
The chibi 3d for trainers I find adorable, especially in XY because the camera doesn't look quite as far back as it does in Oras, and I think that makes everyone look more approachable and lively, and you can customize your trainer in XY. Both Oras and XY are pretty games though, and I think they make perfect foils for eachother with a contrast like night and day. Oras is good at recreating in detail the natural beauty and charm of Hoenn's idyllic landscapes, whereas Kalos is this almost surreal place, full of opulent, glittering cityscapes. Personally I got to hand it to Kalos for it's more risk-tasking designs, clearly they had a lot of fun with it's architecture like Parfum Palace, and the huge sundial in Anistar City, all of Lumiose City, imaginative gyms like the dollhouse one for the fairy Pokemon, and the gym that seems to be floating in space with the psychic pokemon.