>>57586446X and Y were a huge disappointment and have the worst rivals in the entire series (Trevor and Tierno). At the time, it had the "first 3D game" excuse, but retrospectively, it didn't make a huge difference because the following titles had similar problems. It was just even more lackluster. Southern Kalos getting cut entirely makes me even more mad now that Paldea exists.
Black and White were all right, I was a bit burned out on the series at that time so I didn't enjoy them as much as I normally would've I think. Story is above average (which is nice), but in terms of Pokémon designs, it has some high highs and low lows (and I'm the kind of player who likes using new mons and has very little attachment to "muh bros"). I also thought they were a downgrade graphically. I didn't like the overuse of 3D in the overworld, the characters sprites and the tweening animations for Pokémon sprites.
Diamond and Pearl are pretty good. I don't mind the slowness much. I think I like them better than most people my age. To me personally, their biggest strength is the "vibes". I think it's the best looking game graphically, the region has a lot of identity and the music is really good too.