I was just coming to link that. . .
Would you believe, sidescrollers?
Something is wrong with my brain -> hands connection that fails to work fast enough during things like Mario Brothers, Trine, etc. The only one(s) I've ever completed were the original Oddworld ones, which were more puzzle that action, generally.

That my friend, is the best game ever!!!
The game for me is...drum roll.....
Ninja Gaiden! DS version was hard as well, but I beat it on the hardest difficulty. =)

BEST...Mario game ever!

Detroit. I could never figure that game out.

hardest game i ever played goes to the old spectrum game Monty Mole - On the run. could never complete the 1st screen let alone anywere else.

I know what you mean. The game was very confusing for me, because it seemed like shots that only make a small scrape one minute would become a one hit kill the next.

But only the ED Missions... never got past the first one though
The hardest game ever is for me Wizardry IV: The return of Werdna. If you don't know you have to summun priests in the first room to find the exit of your cell you're stuck there before the game really begins. And it won't get easier: Minefields with an invisible way to get trough, dungeon parts cast in total darkness with moving floors and walls (try and make a map of that! Cause you gonna need it!), Ninjas who can kill you with one hit (by the way, most ennemies are clearly stronger than you, and come in whole packs!) and a 3D labyrinth atop the dungeon who reminds me always that "the cube" movie
Ghouls & Ghosts, Ghosts & Goblins are VERY difficult too
I also had quite a hard time with Megaman 9. I finished 2 & 3 (the only ones I got before) but here I could only beat 2 Bosses yet. And masochists can download even harder versions of that game...
I do not count the Metal Slug series as you got infinite continues on that game. But I figure I'll never had finished one of those without these continues!

Yes, and fortunately, Ubisoft implemented a save anywhere feature in GRAW 2 because so many people complained about the lack of it in the first one. Personally, however, I don't think any of the newer ones could touch the original Ghost Recon. (Only game where I called in sick to work to stay home and play. Boy, did I have some explaining to do when my wife got home!)

Men of War.
Although a lot of you are judging whether a game is difficult or not based on how unfair it is to the player. Not so with Men of War. It's bloody difficult, but it gives you the tools to get the job done.
In the second mission of the Russian campaign, for example, you have to defend a large factory complex from repeated waves of German infantry and medium armour coming from three directions, with only sixty-ish infantry, two light tanks and six field guns at your disposal. Your field guns and tanks cannot penetrate the enemy armour (unless you get super lucky). The trick, then, is to use your weapons in different ways: fire HE shells at tank tracks to stop them dead, and create roadblocks. AT rifles can do the same thing. Hide infantry in bushes to throw AT grenades at tanks, or throw molotov cocktails at their engine intake vents. Eventually you get overwhelmed, but it is badass.

The first Spiderman game for PC....I don't know anyone that actually got through the whole game. Spiderman also moved extremely slow which made it very frustrating.
All the hard games that I remember are old ones.....gabriel knight 1 for instance. At least if you play it without using a walkthrough when you get stuck :P.
oh yeah....I want to scream but I have no mouth. Never really even understood what that game was about. You walk around in some nightmare and suddenly you die and have to start over...

without a doubt "the goonies"
i still don't know what to do without searchin on the inet.

Not the most difficult game but without a doubt the most hair tearingly (O.o) most difficult boss I ever fought was the last boss of Dead Rising. My god I hated that fucking guy by the time I was done!

The final boss on Gears of War anyone? I still cant beat his giant ass.
EDIT: grammar check.

Have you tried spanking?

Smartass.

Better than a dumb one :D.