Good thread premise, loads to mention.
One of the worst for me was Pokémon X and Y. I’d been a big fan of the franchise as I was a young kid in the 90s when it became a phenomenon. I’ve never had a present match the excitement I got from unwrapping a Game Boy Color and Pokémon Blue on my 7th Birthday.
After people had accused the formula of being stale - with some justification - the company did a soft reboot with the 5th Generation games Black and White and smashed it out of the park with arguably the best Pokémon games ever, with high quality, moving 2D sprites. X and Y were meant to be the new era, 3D, with 3D models and a more ambitious world. What was delivered was a badly-paced mess where the game looked awful despite a leap in hardware and technology, with the themes of the game simplified, many of the mechanics streamlined and all challenge thrown out the window. Insultingly easy and boring. The only Pokémon game (until I stopped buying them at Sword and Shield) I didn’t bother completing. The franchise has never recovered since and has been awful. Needs a reboot with a longer production cycle and they should go back to high-quality 2D like Octopath Traveller.
Another that hugely disappointed was, as others have said, PES 2008. The start of the decline for the series and it’s arguable it never truly recovered. What people tend to forget is what an opportunity this was for PES, because 6 had been good still and Fifa 07 an unplayable mess that was ported to the new generation of consoles quite dreadfully. Fifa 08 came out shortly after PES’ bodge job and took a stranglehold on the console football scene that has yet to relent. It looked decent, it actually played far more solidly and realistically than a lot of FIFA’s in it’s wake.