Battletoads 1. Original Gameboy.
Flying through those never sufficiently cursed gates that came faster and faster until you had to go from the top of the screen to the bottom of the screen in the exact, or maybe a little less, time that the toad took to move that far. i.e. no time to react, fumble, breathe, press the button, etc.

I think the last one. The first few were easy but the last one was just sooooo LONG. And yes, I get ya about the going so fast you're basically doing it by memory alone. That was bollocks.

VVVVVV.

I'm happy to see that a lot of the old NES games of my glory days are listed here. I've beaten most of the games you guys are saying are really difficult: Battletoads, Contra, Bayou Billy, Ghosts n Goblins, Ghouls n Ghosts, Ninja Gaiden.
I find modern games to be difficult though. I hardly pass anything any more. Largely this is a function of time, but I also just never got that good at first person shooters, which is the dominant genre nowadays.

Yeah traditional platformers are easier in some ways especially not having to do jumps with weird camera angles which is just a killer -.-

Battletoads was definitely a challenge, but I did eventually beat it on the NES without using a game genie or anything like that. The key to beating it was all in the second level. If you smacked the crows repeatedly against the walls, you could get some extra lives. I pretty much depended on those to get through the rest of the game because even when you know what you are doing, it's still hard.
There are two PS2 Contras. Neo Contra and Shattered Soldier. I really liked Shattered Soldier. They didn't hold back. The game is very difficult. I did beat it eventually, but it took some practice.

System Shock on all maxed difficulties.

Life.

You kidding? I suck at hard games, and even I can beat that.

I suck at games, period.

I recall Dark Cloud on PS2 was quite difficult. You couldn't be a wuss and turn down the difficulty on it either. Good times.

Not so much that the enemies were tough, but sooner or later I found it difficult to get the money together to maintain my characters and weapons.

Anyone remember Abadox for the NES? It wasn't the hardest game but it was the most frustrating game. Pretty much you have to beat this game on one life because it's virtually impossible to do so any other way. It's like they give you extra lives just to mess with you and make a mockery of your efforts.

Street Fighter 4...online.