The best looking games imo definitely go to Pokemon HGSS, Pokemon Black/White/Black 2/White 2, Pokemon Ultra Sun/Moon and Let's Go Pikachu/Eevee.
Pokemon HGSS is the prettiest of the Gen 4 games with very clean tilesets, as well as many of the Pokemon sprites being remade and looking lovely. Not to mention of course the 3D visuals being nice too.
Pokemon BW/BW2 stylistically are definitely the most appealing, the mix of 2D and 3D elements from the backgrounds alongside overworld sprites to in-battle 2D sprites (one of the reasons why the game has aged so well visually compared to other DS games of the time) on contrast with the 3D battle grass below the Pokemon is very lovely to look at. One of the most visually appealing DS games too!! Does what HGSS but amplifies it by 10! :D
Gen 7 was imo where Pokemons 3D design was done consistently well. The environments felt real, for the first time they were also to scale and it was nice seeing all the Pokemon with their big 3D models unlike XY where most of the time their model was cut a little because of battle perspective. The battle backgrounds in USUM are also super gorgeous, the Route 1 one with flowers is definitely a highlight!
In terms of graphical fidelity Let's Go is the prettiest because of how consistent it's graphics are, as well as how much the colours pop and the nice animations concerning flowers in the background and the water!! This isn't to mention the following Pokemon which were also done nicely in terms of graphics (thinking of Snorlax ride hehe).