Kingdom Hearts Birth by Sleep.
It effectively spelled the beginning of KH's decline as a video game and not a cutscene simulator. KH2 may not have been perfect, but with FM's improvements and additional content with Critical Mode, it was a very good action RPG with solid combat even if it was a little simplistic at times. BBS was even more of a button masher as basic attacks sucked terribly and sometimes could get you hit/killed for using them, bosses could just randomly tell you to go fuck themselves (See Mysterious Figure), bad game balancing as most commands you could use sucked, and Terra was actually not balanced for BBS' boss design because his dodge ability doesn't let you have infinite I-frames by mashing the square button like Ventus and Aqua's dodge ability.
The characters introduced were a waste of time even when we got KH3. Beyond Xehanort, no one important was introduced and Xehanort didn't even need to be introduced this way or involve Terra, Ventus, Aqua, or Eraqus at all. What an absolute waste of these characters and a waste of a game. A lot of design problems would plague future KH games because people rated this game's combat better than KH2 because somehow mashing triangle to do any notable damage ever and mash square to I-frame through everything is dramatically better than "the mash Triangle and X to win" mentality KH2 left even though FM had super bosses that would push your shit in if you tried just mashing X especially on Critical Mode, but we'll pretend those don't exist even years after we can actually play KH2FM in full English. Even the hardest fights in BBS boil down to dodge roll until you get an opening, press a couple of commands, cure if you need to cure because curaga is almost always up when you need it, and repeat until you win.
An example of how no one play tested BBS with Terra especially in Critical Mode, note that Ventus and Aqua can basically mash square vs this bullshit and never take damage so you can attack during the limited times it is safe to whenever they want.