I'll cast another vote for "I Wanna Be The Guy."
Also, did anyone ever play Zelda 2: the Adventure of Link?
[url=http://en.wikipedia.org/wiki/Zelda_II:_The_Adventure_of_Link]http://en.wikipedia.org/wiki/Zelda_II:_The_Adventure_of_Link[/url]
All the old 2D sidescrollers are hard, but this one was downright murderous. I never managed to get past the second dungeon.

Any of those really old dungeon crawlers that auto-save your progress. Rogue and Wizardry come to mind.

Airwolf wasn't that bad. I managed to get to the end and back a few times as a kid and I'm sure some of my mates could do the same. Having said that, to beat the game you had to do it about 5 times in a row which I certainly never got anywhere near.
If you want a really hard speccy game how about the Jet Set Willy clone Technician Ted. There were a huge number of rooms and you had to complete the 21 tasks in a given order (except you weren't told what the order was). All the rooms kept changing as you played, loads of the jumps had to not only be pixel perfect but also timed perfectly and to top it off you had a time limit so harsh that even if you played through the entire game never making a mistake during 50 minutes of gameplay you would barely scrape there in time. Its a game so hard you would have to dedicate years to beating it without cheating.

Another recently-discovered inglorious-bastard is this shitty little colourful flash game. Please try to go beyond level 7 (Medium difficulty) and let me know if you succeed....

Yeppers, you're right. I just checked Moby, last Shinobi game was for ps2. I think Alias is talking about the recent XBLA re-release of Shinobi, no? That was recent, I think. Maybe I'm wrong.
I thought for a second they made a new one after Nightshade (ps2), too. Dear God, that game was hard.

Battletoads once you get to the turbo tunnels....

Battletoads, period. God, that game was outrageously hard.
Oh I have another one. Bart vs. the Space Mutants. Egregiously difficult. It took me hours to figure out I had to inexplicably spraypaint things purple to stop the space aliens. Also, TMNT. That game was a hairpuller.

TMNT was easy - just use Donatello for everything except the water level where you have to swim and disarm the bombs. Donatello's staff weapon had the furthest reach out of all the Turtles and he made the game easy; the other turtles were water level fodder...

Talking about simpsons games anyone ever try Bart's house of weirdness? I thought that was way harder then Bart vs the space mutants. I think after weeks of gameplay I managed to only beat one of the levels.
[url=]http://www.mobygames.com/game/dos/simpsons-barts-house-of-weirdness/screenshots[/url]

Oh, and btw I've just remembered that no, Syndicate - American Revolt isn't the most difficult game I've ever played.
This title should go to Metal Slug II, in the arcades. Its difficulty is simply amazing, so much that I still remember it after more than a decade since I played it for the last time...

I Wanna Be The Guy. As far as retail games go though, my vote is for The Adventures of Bayou Billy (NES) and Ninja Gaiden (NES).

I remember that. I only managed to get to the first boss, the one where Maggie helps you. It really is very difficult, you have to time your movements properly throughout the game. One wrong move and you have to start all over again.