It's an interesting question, because I think "difficulty" is much different now than it was say, back in the 90s before the Internet and walkthroughs. I'm an adult, my free time is fleeting, so I have no qualms about using a walkthrough if I'm stuck on a section in a game even if I feel retarded afterward for not figuring it out after 10-15 minutes of fumbling around. Even games that are infamously difficult like Ninja Gaiden Black can be figured out with enough time and recognizing patterns of bosses which can be done by watching walkthroughs, since you can rewind, slowdown, etc. while watching and with enough practice and pattern recognition, you will beat that boss. And with games that can be emulated, there's savestates and rewinding.. Which, you can argue that's not how to "truly" beat a game, but it is an option and if I don't want to play the same goddamn stage of Super Ghouls n Ghosts fifty times to get past it, I'm using savestates.
So with all that context in mind, the only game that I can think of that is
truly difficult even in 2025 to a byzantine degree, is
Drowned God. I think the main reason why it's difficult at least for me, is because at its core, Drowned God is a Myst-like point-and-click adventure game,
but made with the moon logic of a legit schizo.
I'm terrible at solving puzzles already, even though I do enjoy a lot of point-and-clicks. But I think even for people that like puzzles, and aren't logically challenged like I am, would have a difficult time with Drowned God. Even with following a walkthrough, the game's internal logic is really dense and takes a lot of time to unravel. There are layers upon layers of references to ancient religion, old books, X-Files conspiracies, history, and a bunch of other shit that I feel like you need a textbook just to understand what the fuck is going on half the time. I can't really say if it's a good game, but it is a fascinating game, and one that I think about a lot. It was also made by a guy that went schizo and killed his family... or might have been murdered by someone or some other entity entirely, to this day no one is really sure. And of course,
sloptubers have made videos about that, but I find the game Mr. Harry Horse made much more interesting, and even more confusing. Check it out: