I think most people agree the series peaked before gen 6.

Gen 5ers praise the post game content there, but I find it the breaking point for me where too many of the designs didn't "feel" like pokemon anymore. Though, they a stiil far better than all the furry bullshit that came after. I give people who consider it the best a pass.

For me gen 4 was the best. The physical/special split added a lot to the gameplay. On the other hand, the interface was the slowest it ever was and stealth rock and weather were overcentralizing. Still, I find these issues less severe than some that other gens had.

Gen 2 also stands out, due to its focus on refining the formula of Gen 1. It lets you experience the classic pokemon formula without the severe gen 1 jank. Some of the best experiences to be had on the original gameboy.

The fact of the matter is, no one thinks pokemon is at its best right now, except small children (and people so easily swayed by marketing they may as well be children.) We debate whether the game was last great 15 years ago, or 20. These days, the gameplay changes are gimmicks, the worlds feel smaller and smaller, and the increasingly humanoid pokemon designs feel like they're pandering to furries, and make the whole capturing and making them fight mechanic feel more unpleasant. Modern pokemon feels like a one of those monster catcher copycat games that were so popular in the 90s somehow got the official license.