Gen 4, specifically Platinum and HG/SS, were the peak of the series. But I think there were also some design choices made in Gen 4 that sowed the seeds for the series' decline.
B2/W2 were very good games too, but the rot that crept in during Gen 4 had clearly started to bloom.
Gen 6 as a whole was low effort garbage. I still believe that X/Y were supposed to have a sequel that was cancelled to make way for Gen 7. Could it have saved Gen 6? Probably not, but, I dunno, maybe.
Gen 7 was better than Gen 6, but I still didn't really enjoy it. At this point, I'm tired of the gimmicks. I'm tired of almost half the fucking Pokedex being legendaries. I'm tired of how mind-numbingly easy and linear the games have gotten.
If Gen 8 is more of the same, I'm done.