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Gen 1 and 2 were the peak of the series's artstyle in terms of official art (Not ingame sprites in Genwun's case we all know those were really bad, though Gentoo has nice sprites).
However, as far as the actual games go, Gen 1-5 are a logical progression of increasing quality. Gen 5 are the series peak gameplay wise.
Then Gen 6 happens, and the gameplay goes to utter shit, followed by the art direction when Gen 7 happens. And yet, Gen 7's art direction looks like fucking Michaelangelo compared to Gen 8's art direction. Jesus Christ, I like Turner's pokemon designs from the previous generations, barring Vanillite sorry Vanillite, but I really do not like him as art director.
tl;dr Genwun/Gentoo official art is peak aesthetic, Gen 5 are peak gameplay, the newest games are fucking terrible in both regards.
Pic unrelated, it is the cover art of the 2011 Nintendo DS game, Kirby Mass Attack.