Hardest in a different way than you probably mean for me would be This War Of Mine. One of the first missions you basically have to stab an old man and his wife after you broke into their house. It’s rough.

Iwbtg and all its successors and fan projects

The Donkey Kong tilt minigame in Nintendo Land is brutal.

The hardest one I can say I’m honestly proud I figured out are the old “Impossible Mission” games from Epyx.

They have set rules, and once you figure out all the rules, they are solvable, but the platforming elements require precision and the puzzle elements are challenging.

8-bit Souls-Like?

Longplay - 1:

https://youtu.be/ivHFP3dJAkM

Longplay 2:

https://youtu.be/O2AEuLjwBrg

Army Moves, Navy Moves, or any other old Dynamic Software game. You’d have to be very skilled to get out of the first stage.

Probably “Trap Adventure 2”.
Imagine an old Mario game where Bowser has the most rediculous traps set up. You need to memorize all of the trap locations as well as have the coordination to tip-toe around them to survive.

https://www.youtube.com/watch?v=_nW9k6k1I3k

Crypt of the NecroDancer.

There are three big challenge characters in the base game:

• Aria can only use the starting dagger, no other weapons. She has only one hit point. And she dies if the player ever misses a beat.
• Monk dies if he picks up gold. All enemies drop gold, even ones that normally wouldn’t, which turns the game into a routing puzzle where you must never step on squares that an enemy previously died on.
• Bolt plays the whole game at double tempo.

Once you have beaten these three challenge characters, plus the other six easier ones, your next task is All Chars Mode. Beat the game nine times in a row, once with each character. If you die, you must start the whole marathon over.

Beating that unlocks the tenth character, Coda. Coda combines the restrictions of Aria, Monk, and Bolt all at once.

And if you can do that, the final achievement is Lowest of the Low, which requires you to beat All Chars Mode without collecting any items.

The DLC adds a few more hard characters, and another achievement for an extended 13 Character Mode, but they aren’t considered to be as hard as Coda or Lowest of the Low. A single digit number of players have stacked the challenges for Coda low% and 13chars low%.

I always put the original Blaster Master on the NES up there.

It had no save capability at all, nor any codes to stop & restart later. When you sit down, you better be ready to do the whole 4+ hours in one playthrough (or just leave the NES on & walk away).

But the kicker was that once you got hit just a few times, you might as well restart. The gun (in person mode) would power down with each hit, and after a few hits, well, you just didn’t have enough ‘oomph’ to kill the bosses. But the power-ups to get the gun were fairly sparse in the first place, so once you got hit, it wasn’t like you could just retrace your steps & power up again.

Mildly interesting, at least to me, I understand it’s been remastered for the Switch. It now has save points AND being hit doesn’t reduce your gun’s power. That would make it a completely different game. I’m be curious to check it out someday. If nothing else, I’m curious to see how much of it I remember. I suspect I can autopilot the first 2 hours, despite it being 40(?) years later.

The original Prince of Persia

Out of the games I’ve played, OSU. I am pretty average at rhythm games where it’s like Project Sekai or the Miku Diva style games where all you have to do it wait and click a button or tap somewhere specific at a fixed location on screen, but I absolutely suck at the whole move the mouse and click thing. Just as bad with mouse as when I tried with my beginners tablet.

Most other games I play anymore are games I know I’m at least decent at, so I don’t have many games I’d consider the hardest or even to compare those too. Though, while writing this and thinking about it, I’d say I might compare OSU to Vib-Ribbon in general, default songs or not, and possibly even give it a close second for difficulty. And that’s despite it being more of a wait and click type rhythm game in my eyes.

Mushihimesama Futari was the hardest game I’ve ever played, and this is coming from someone who enjoys playing Touhou on Lunatic Mode. My god, the bullets, the sheer amount of bullets! I felt so proud of myself when I finally beat this boss.

The classic arcade game Venture. Go ahead, make my day:

https://archive.org/details/arcade_venture#

Venture is a 1981 arcade game by Exidy. The goal of Venture is to collect treasure from a dungeon. The player, named Winky, is equipped with a bow and arrow and explores a dungeon with rooms and hallways. The hallways are patrolled by large, tentacled monsters (the “Hallmonsters”, according to Exidy) who cannot be injured, killed, or stopped in any way. Once in a room, the player may kill monsters, avoid traps and gather treasures. If they stay in any room too long, a Hallmonster will enter the room, chase and kill them. In this way, the Hallmonsters serve the same role as “Evil Otto” in the arcade game Berzerk. The more quickly the player finishes each level, the higher their score. The goal of each room is only to steal the room’s treasure. In most rooms, it is possible (though difficult) to steal the treasure without defeating the monsters within. Some rooms have traps that are only sprung when the player picks up the treasure. For instance, in “The Two-Headed Room”, two 2-headed ettins appears the moment the player picks up the prize. Players die if they touch a monster or the corpse of a monster. Dead monsters decay over time and their corpses may block room exits, delaying the player and possibly allowing the Hallmonster to enter. Shooting a corpse causes it to regress back to its initial death phase. The monsters themselves move in specific patterns but may deviate to chase the player, and the game’s AI allows them to dodge the player’s shots with varying degrees of “intelligence” (for example, the snakes of “The Serpent Room” are relatively slow to dodge arrows, the trolls of “The Troll Room” are quite adept at evasion). The game consists of three different dungeon levels with different rooms. After clearing all the rooms in a level the player advances to the next. After three levels the room pattern and monsters repeat, but at a higher speed and a different set of treasures.
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Released
1981

Probably some of the old Nintendo games. Silver surfer is an extremely difficult bullet hell. Battletoads required insane memorization and timing, pretty sure you had to act before the game even told you in some places.

Battletoads

QWOP, by a wide margin