What is the hardest Dungeons and Dragons module?

Tomb of Horrors is designed for players of levels 10 to 14. Depending on how often you play and how your DM handles the distribution of experience points, getting a character to this level can take well over a year of play. 17-Mar-2019

https://www.eurogamer.net/inside-tomb-of-horrors-the-hardest-d-and-d-module-ever-made#:~:text=Tomb%20of%20Horrors%20is%20designed,over%20a%20year%20of%20play.