From Cuphead to Super Meat Boy: 10 of the most fiendishly difficult games
theguardian.com
theguardian.com
Dungeon Crawl Stone Soup (a rogue-like.) You could play this for months and not beat the game.
ROM hacks like Kaizo Mario and Super Metroid Impossible.
Playing normal games blindfolded: https://m.youtube.com/watch?v=ZcOd82Q6-oU