QWOP is just "Shitty controls: The Game". Which is just artificially difficult.
I mean, if you want something that's hard to beat because it's shitty in general, just go play Cheetahmen (which is much harder than QWOP, if only because the game has a chance to randomly crash every time you jump. Hey games of chance are still technically games!).
If you want games that are hard because they were made that way (and well made), I Wanna Be the Guy and innumerable ROMhacks will satisfy your need for self-flagellation better.
Keep in mind that there is no definitive "Hardest game ever" because any game can be beaten through rote memorization or (in the case of shitty design) random chance. What games bother one player might seem easy in comparison to another, depending on what one has played most often.
So far, the most legitimate hardcore gaming accomplishment I've seen to date is HideofBeast's Minimalist Run of Megaman X6 (and I think I just heard every MMX player out there yelp in fear). It's so much worse than I Wanna Be the Guy (which I've beaten. I cannot even come close to pulling off half the insane shit required to beat MMX6 in this manner).
NOTE: These are NOT tool-assisted-speedruns. What you see is the result of a guy slamming his head against a wall for months. His post-run commentary is totally priceless.
Link:
http://www.youtube.com/user/HideofBeast#p/c/9A7DD8572486C671
(not embedded because it's a page, and this is far more convenient than trying to hunt down his videos one-by-one)