The first five Pokemon generations had fantastic pixel art. Each generation had its own distinct style and felt like a solid step up from the last.
The fifth generation (Pokemon Black/White) was unique in the sense that it was the first generation that used the same hardware (the Nintendo DS) as the previous generation, so they had a chance to get more experimental by using a pseudo-3D overworld and fully animated sprites during battle to help differentiate those games from Diamond/Pearl/Platinum.
After that, they attempted to make the transition to full 3D for the 3DS era, which is where the series really fell off a cliff. Each new generation feels less like a new world full of distinct handcrafted sprites and more like a poorly-shaded mess of recycled 3D assets that somehow struggles to maintain its intended frame rate.