Prince of Persia: Warrior Within is always the first game that comes to mind when I think of disappointing games. Sands of Time was great. The combat sucked, but it was very aesthetically pleasing with beautiful environments, a really awesome arabian-rock soundtrack, and just a great atmosphere in general. The story was pretty decent as far as video game stories go. (It had a good ending.) Then came Warrior Within. Holy crap. They took everything that was good about the first game and made it cheesy and bad. Changed the music to screamo, made the sand creatures bleed for some reason, made every female character super slutty, and even changed the Prince's accent for some reason. They "casualized" it, back when "casual gamers" were people who played dudebro shooters and madden. It was also a glitchy mess. My first time playing, I had to restart after an hour or so because I got turned into a Sand Wraith too early and couldn't progress. The best part was that they worked so hard to fix the combat and I still ended up using the same two moves on every enemy. At least The Two Thrones was pretty good. A return to form.

Runner up is No More Heroes 2. It's too dark. The first game had pretty poor gameplay but a lot of charm in the characters and cutscenes. The second one was much better to play, but kind of soulless. That one made me sad. I didn't play No More Heroes 1 for the gameplay (though the boss battles were awesome), I played it for the insanity.