Dark Souls got the most mentions, with 14% saying it was the hardest game they've ever played. It was followed by Dark Souls 2, which took in about 5% of the results. From there, though, the results are immediately diverse, with shooters, platformers, puzzle games, strategy games, and MOBAs all bunched together. When I cut out jokes, console games, games with specific caveats, and those that received only one or two mentions, I was still left with over 70 games. (Here's my curated list of the top 77.)
Further down we find StarCraft II, STALKER, Insurgency, Alien Isolation, Kerbal Space Program, the Touhou series, VVVVVV, Volgarr the Viking, SpaceChem, Dustforce, Commandos: Behind Enemy Lines, and, of course, Bad Rats, a notoriously awful game which has accrued a positive rating on Steam, because ironic Steam reviews are all the rage.