There were so many games in my youth (80s-early 90s) that were difficult simply because the games were badly made. That persisted well into the early 2000s. I'm also not going to go with a game because it's got a "are-you-f'ing-kidding-me" difficulty setting. So discounting all that, I'd say... Dark Souls 2 DLC. While some people really liked the DLC, I found it uncharacteristically difficult, even by Souls standards. I'm not the type that does no bonfire NG+7 runs or anything, but I've been able to clear all the other content up to the ziggurat in the sunken king and I took a break after finally downing Aava in the ivory king and haven't been back since. I'll keep plugging away at it eventually. I'm addicted to Soulsborne in a bad way. Edit: I did get the Fume knight down, but I haven't done the blue smelter demon yet.