What is the most difficult accomplishment in a video game you've achieved?

Whether it's beating a very difficult game, getting 100% on a game, getting a speedrun finished, doing some sort of challenge run, etc.

I'm not really the type to go for 100% clears a lot, but I enjoyed Kid Icarus: Uprising so much I was compelled to get all the Achievements without using the skips. Towards the end of that I was genuinely worried it wasn't healthy... (the second to last level on the hardest difficulty is absolutely brutal).