The title says it all: what is the most difficult game you have ever played/are playing? (searched on the forum, found nothing related...)
For me until yesterday I thought it was Ghosts'n Goblins and this sort of things, but now I've changed my mind....
Syndicate - American Revolt is the hardest pain in the ass I've ever played, I mean, the original game has been a nice playing through the last month (July), with some tricky missions and the infamous "Atlantic Accelerator" final to end it with a true bang. But this damn data disk is disgustingly difficult, I barely succeeded to go as far as the beginning of the third mission and now I'm stuck after having restarted it countless times.
Well, I love challenges, but this seems sadistic to say the least. Damn Bullfrog :-P

I Wanna Be The Guy.

Yeah, what a perversion :-D

Definitely IWBTG.
Otherwise, Rick Dangerous.

This one whatever it is. I never could get past level 1 when I tired it a good while back.

I declare this Game of the Year.

American revolt isn't THAT hard, I finished it after all.
Hardest game I'm playing at the moment is Shinobi on the 360. Somewhere towards the end of the game they decided to spam you with instant kill ninjas who can jump like bloody superman and can spontaneously materialise and kill you as you're walking along before you really have time to process the fact they're there. Its definitely a remnant of the coin vaccuum arcade design mentality. The worst part is the 360 version accurately simulated a kid with sod all pocket money so you only get a very small number of continues
Paradroid is another in the really hard basket. Not because of the difficulty of the game itself, that perfect balance of fun & challenge was one of the true joys of it but its the sheer endurance. If you wanted to clear one of the freighters completely (and I think you needed to in order to progress), it took 3-4 hours. The manual said there were 13 ships in the fleet but according to wikipedia it resets after you clear the 8th so there IS no end! Its a game that REALLY needs a modern remake though, that game could rock so hard if redone with love.

Rick Dangerous 1&2 are just frustrating. It's just a matter of patience and memorising where the traps are, if you can be bothered to do that and don't mind restarting the level quite a few times then it's completable fairly quickly.

Unplayable - Mistmare, Superman 64. Oh, God what disasters. Mostly difficult due to poor coding and weak design, rather than any inherent difficulty in the scenario design. Mistmare earns the dubious distinction of being my least favorite RPG of all time. I ejected the disc and lit the game on fire after playing it for a half hour.
Playable - Of late, Breath of Fire: Dragon Quarter (ps2). Ungodly hard. Much of the team worked on Dead Rising. Imagine the same save system used in Dead Rising in an RPG. Shinobi (ps2) stands out too. The final boss in that game was ungodly difficult for a modern game. Well made game, but too hard for me - I'm not that good at action games..
Alias, how's the new Shinobi? It can't possibly be harder than the ps2 one, can it?

I'm a wuss who gets frustrated whenever I can't beat a level, but the hardest levels I've ever come across (or more like, the ones I've played over and over again and somehow didn't quit along the way) have been the final level in Psychonauts (the meat circus. I'm simply not a 'jump at the right time' kind of guy) and the last level of the american campaign in Original war, which took me many, many replays until I found a way to overcome it.
If a game is too hard I'll usually think it's ME being an incompetent player and simply stop playing them but those two levels managed to somehow keep my interest despite multiple reloads.
I'm not a console gamer and my game collection isn't that big, so these two might seem like they aren't hard at all for the hardcore gamers among you.

For me its Mystery Dungeon: Shiren the Wanderer for the DS. Its a roguelike game that when you die you go back to the beginning.

The Original Bard's Tale on the Amiga was a complete b*stard.

Airwolf on the zx-spectrum.
I don't know if anyone ever completed it on the original machine, or just on emulators where they had the ability to slow the game down.
I must have played it dozens of times and never made it past the first force-field. Never even got as far as an enemy!
Turned out it was only about 4 screens long... but they made it so hard that i think very few people ever found that out.

Ninja Gaiden on the Xbox comes to mind.

Super Star Wars series on SNES.