I have an obsession with playing really difficult games just for the challenge. Since I am not a really good gamer, it takes me an obscene amount of time to beat some of these games. The most difficult game I have ever played is Sekiro (from the creators of Dark Souls):
I am proud to say that, after almost 6 months of playing for about 30 minutes every couple of days, I have finally beat this game. I probably only died a few hundred times (not exaggerating).
The difficulty definitely was the addicting part of it for me. Some of the boss fights required learning all of the move sets for the enemies perfectly and timing your appropriate attack perfectly. Ninja Gaiden was similar in this way, but Sekiro makes Ninja Gaiden (even at its Hardest setting) seem much more tolerable
If you have the time, you should give this game a shot @tbo!