Final Fantasy XIII's ending was definitely a game ruiner.
I didn't mind the story being difficult to follow or the linearity of the progression or the repetitive battles THAT much. Those things were annoying, but the occasional good boss battle or good plot revelation kept me interested enough to see the game through to the end, and I was confident that the ending would give me some fun and challenging boss battles and an at least decent payoff story-wise. I could not have been more wrong.
Gameplay-wise, the final boss has a ridiculously cheap "instant game over" move that you can only stop if you have very specific and not easily-obtained equipment. This is bullshit for two reasons: 1) There's no way to know that you will need these items other than to invest 10-20 minutes of playtime into the final sequences, get killed, and then go back and get what you need. Depending on how much loot/money you have knocking around spare, you may be required to backtrack and grind, adding 3-5 hours of work to be able to win the battle. 2) The equipment system in the game is EXTREMELY streamlined and designed to minimalize its presence throughout the game. Then this system does a 180 degree turn at the end and says, "Hah! You can't finish the game unless you have these extremely specific pieces of gear!"
Story-wise, the ending ridiculously contrived, nonsensical, and hardcore "deus ex machina." Even by the standards of "contrived nonsense" set by JRPGs in general, FFXIII's story ending is STILL batshit-crazy-stupid.
A good ending could have redeemed the game up to "decent" for me, but instead, the ending dragged the game down to Satan's basement.
I'm very gentle with the physical copies of my games, because I want to make sure the disc never gets damaged and the box/cover art stays in good shape. Even so, Final Fantasy XIII remains the only game I have ever been so disgusted with that I picked it up and threw it.