Since the topic is "hardest game you've ever played" and not "hardest game ever made", I'll forgive everyone for failing to mention Dr. Jeckyll & Mr. Hyde for NES. In fact, I'll commend everyone for never having played the game and strongly suggest that you never even consider doing so.
Dr. Jeckyll & Mr. Hyde is the hardest game ever made. I played through it several years ago in the process of writing an article about the game. Using the Game Genie code that made me invincible, it still took several hours to play through the game. In fact, in one of the later levels, it took me over 5 minutes to make the equivalent of one screen's width of progress WHILE FUCKING INVINCIBLE. We're not talking about some puzzle dungeon or some shit, we're talking about side scrolling levels with no environmental traps (ie. holes) and I still couldn't do it.
I'm actually in the process now of putting together a documentary about the game. As far as I've been able to determine, it was designed by a Saudi Arabian company. My hope is to bring this company up on charges of terrorism. Failing this, I would at least like to punch one of the programmers in the face.
Even worse, the game has really nice graphics for an NES game. If you've played Just Breed, I would compare the two graphically. As far as play control, however, there's nothing in Dr. Jeckyll & Mr. Hyde that couldn't have been done on the Atari 2600, except I've never played a 2600 game with that shitty of play control. (And, on a random note, I bought two copies of E.T. for the 2600 yesterday for $2/each. I'm open to suggestions on what to do with the extra copy. Shooting seems a bit passe, I was contemplating gluing it to a dildo and selling it on eBay.)
But back to Dr. Jeckyll & Mr. Hyde. The game fucking sucks. I mean, it sucks more than anything else on earth has ever sucked. I've known Jewish elders who, upon playing it, commented that maybe Auschwitz wasn't so bad after all. I've contemplated shoving the cartridge up my ass just to prove that doing so is actually more enjoyable than playing the game, but my poop really deserved better than this horrid pile of suck.
As for Street Fighter 2010... I don't think that game was so much hard as it was crappy. I don't think I ever made it past the 3rd fight, but mostly because the play control was utter shit. Also, did anybody actually get this game NOT thinking it was like Street Fighter 2?