Okay thanks everyone so much! I... wasn't sure what I was expecting to see in the replies, but I definitely had some other games in mind. I was thinking more along the lines of rhythm games (yes IIDX/SDVX I'm looking at you, no I still can't consistently clear lvl17 on SDVX), since most rhythm game feature levels that are just downright humanly impossible... but I assume the JP-based rhythm games are way too niche for most people, and Guitar Hero/Just Dance aren't too difficult in the grand scheme of things

I guess it makes sense that for many people the most difficult game would be some bizarrely difficult game from the 80s/90s since... I thought the rationale for making a video game challenging is to make it more replayable & create the feel of having more "content"? Games back then literally don't have the technical ability to create a 40+ hrs unique gameplay, so I guess until roguelikes/roguelites became popular it is a good strategy to just make the game really hard (which also coincides with arcades' need to make more money from ppl failing more). Which I guess makes From Soft games quite interesting since they are challenging despite having no lack of gameplay elements in the games themselves

And speaking of roguelikes/roguelites, I guess if people were to base the difficulty of a game on "how many people could win a run", "how long does it take to git gud", or "how consistently can a reasonably experienced player beat a run", roguelikes/roguelites would top the charts on most difficulty rankings... which I find kind of funny

I also have a personal hypothesis that for any action-based games, people find games with more "abstraction", i.e. the control scheme is more unintuitive or far-removed from the player, difficult. For example, a 90s platformer would feature you pushing buttons on a controller, which then feeds into your screen character moving while being influenced by game physics, which is an absurdly high amount of abstraction... whereas a game like Fruit Ninja has close to zero abstractions (you literally just swipe the fruit) and would probably be considered quite easy by most. Obviously doesn't apply to non-action based games but I think they are the minority among all video games

But honestly, I know I'm asking for difficult games here, but I find even just the 1985 Super Mario Bros quite challenging (mostly because of the jank physics engine but more about that another time)... games from that era truly are something else. And this is speaking from someone who had 100%ed or otherwise fully cleared many popular roguelike/roguelites so...

Anyway I think the short conclusion I had is I should play a few retro games that I haven't had a chance to try yet. Oh and traditional bullet-hells. Just for shits and giggles... thanks!