Generation V is an obvious winner for me, but I won't get into why, because I'm sure everyone knows why.
So I'll instead talk about Generation III, whose aesthetic is a runner-up in my eyes. I'm not sure why, but I just love the artstyle, especially on the Pokémon sprites. I'll just post a few sprites to show you what I mean: the left one is from Generation III; the right from Generation IV.


I'm not sure what it is, but the sprite on the left just looks more appealing to me. Sure, the right sprite is obviously more polished because it's not as limited in terms of color or size as the left sprite, but I don't know. I just like the
style of the left sprite, particularly the shading. Here's a more drastic example:


Arbok and Skuntank have somewhat similar colors, but Skuntank's sprite definitely has less shading than Arbok's. I lowkey wish they kept the Generation III shading style in Generation IV (which they did during development, as seen below, but changed later on).



You can see that Weavile's and Munchlax's beta sprites have more shades (or at least, their shades are much more visible) on them than their final sprites. I can see why people would prefer the simpler sprites with more refined colors and less rendering detail, but the GBA games' artstyle is just eye candy to me.