Honestly, Armored Core VI. Endgame spoilers below (idk if there's a way to do spoiler tags?).
The final boss is absolutely godawful. Just utter garbage. It took me hours, and I hated it from my first attempt. It's categorically different from anything else in the game, and there's never a point where it's fun. Probably 20% of my total playtime was on this one boss. I was absolutely loving the game up until then, but that one boss is so unbelievably poorly designed that it ruined the entire game for me. It's genuinely impressively horrible.
(Spoilers continue)
I don't know for sure, but I think it's just a couple missions before the end. You have a choice to either fight Cinder Carla (siding with Ayre) or to fight the corps (siding with Carla). I sided with Carla, which made Ayre the final boss, and the fight was godawful. My understanding is that there are maybe more endings with NG+, but I'm trying to muster up the will to bother even turning the game on again after how atrocious the Ayre fight was.