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.