Demon’s Souls, the Dark Souls trilogy, Bloodborne, Hotline Miami and FTL: Faster Than Light are all the hardest games I can remember beating off the top of my head. There’s something so satisfying about dominating and overcoming a tough game. Personally, I don’t use cheats or anything like that, but I do sometimes read a guide/tutorial if a game is particularly difficult and I want to get past a certain section.