what's the hardest video game you've played in terms of base game difficulty (on the hardest difficulty available) rather than a game that simply has a high skill ceiling?

hard mode: no dark souls games

They bleed pixels, touhou, jamestown and ToME
TBP is probably similar to SMB's difficulty. It was really hard to beat the game on the first playthrough.
Touhou and Jamestown is silly, bullets everywhere and such.
ToME is something you have to be absolutely insane to beat on the highest difficulty like i don't even know normal is hard enough (read: too hard) for me.

2 games already told by Lazors, Touhou(not that fighting one its easy, but the danmaku one) and ToME.
For me bullet hell games are always a difficult games, even I only able to 1cc on normal difficulty and lower in most Touhou danmaku games (once I 1cc'd on hard, but it was Imperishable Night which is one of the "easiest" Touhou danmaku games).

Another bullet hell game which I think difficult is orange_juice games, Suguri and Sora(Suguri is more difficult in my opinion), where you need to move pretty fast to dashing through some bullets or dodge them.

ToME, well, it's a rogue-like so you're gonna need to be lucky when playing it.

Hotline Miami, where every enemies able to kill you in one hit/shot.

Super hexagon is one
And maybe geometry dash.

I like fast reaction games

I think it's not fast reaction it's memory game? I used to play this kind of games before when I was a kid and it's 99% just remembering the level right? Neither of them look randomly generated maybe I'm wrong, is it different with every run?

If you want reflex games then play a twitch shooter like cs go.

+1 for Super hexagon.

Such a simple game, yet so difficult and intersting. The soundtrack is pretty damn neat too.

Last level is the only one I couldn't beat.

The 3d touhou are ridiculous too, most awkward thing I've ever seen.

Regular touhou is hard tho, probably the hardest game aroudn atm.

Mega Man 2

Super hexagon is randomized but you can pickup the same types of 'shapes'. geometry dash is pretty much memory though.

also for me dark souls 2 scholar of the first sin, i dont pick up many difficult games but this was the only one i did choose to buy so compared to games like warframe, naruto, and mirrors edge its pretty difficult. still havent completed it. other than that tho i guess one finger death punch for me since its pretty fast paced and after a while your fingers give in

how the fuck do losers sexualize touhou?
for the sake of staying on topic, a lot of snes games are nearly impossible.