Gen 1: As someone in fan-made game said it best, the types were unbalanced and the designs were bland. : D
Gen 2: The variety of Pokemon to catch isn't really a lot IF you take out ALL Pokemon that weren't introduced in this generation. The game honestly relies on Kanto a bit too much, to the point that it's included in post-game.
Gen 3: Too much H20 to be honest.
Gen 4: Lack of available Pokemon to catch, but Platinum fixes this. My favorite gen.
Gen 5: This one is REALLY nitpicky, but the main character designs are kind of bland. Same for B2/W2.
Gen 6: Fairy type is dumb imo, at least with the way it was handled. Bonus: Mega evolutions weren't given to a lot of Pokemon that actually needed them.
Gen 7: The handholding and long cutscenes kill this for a lot of people.
Gen 8: The story was actually kind of terrible compared to the other games, and the characters felt bland. Hop is also a pushover as a rival, just like Hau. Bonus: Unwanted Charizard fanservice in the form of Champion Leon.
Gen 9: The glitches that could happen in Tera raids are annoying.