Playing theworldshardestgame now, yeah level 10 is... yeah. Hrm. Much smaller window at the 6th ball. Much smaller. Yeah.
One of the most difficult things I've done is the Secret Message shinespark-charged marathon in Metroid Fusion. I know Metroid is big around here but that was just so hard.. I have big thumbs so I hate tiny little D-Pads. I wonder what it would be like on a GC controller. It took me weeks of practice. Gave up. Came back to it a month later. More weeks. One morning I got up at 6 am and finally pulled it off. I woke up everyone in the house...

One odd yet incredibly challenging game I've played is a frogger clone UMS map in Starcraft. My friends and I played it for hours and hours, eventually one friend and I were able to beat the last level (5). Every time you die it spawns like 30 zerglings around you and kills them all. Splat. Sometimes the AI would get a little screwy and they'd veer off their destined paths. Especially the reavers.
Fuck those reavers.
Oh! Jumper 1 and 2! Flash based game where you're a little red box named Ogmo or something. You have the arrow keys, a double jump, and that's it. Well in Jumper 2 you can do a side flip and a wall jump, but still only arrow keys. I got all the gems in Jumper 2 and spent awhile speedrunning the first 5 worlds, actually. Some guys were competing on a forum (I think it was samus.co.uk) and I was matching or besting the lowest listed times. Got burnt out after world 5.
As for entire commercial games..
-
Legacy of the Wizard on the NES. To be fair I was pretty little at the time but this game was a big goddamn mystery to me. I don't think I even had a manual. After watching the speed runs I see just how far in over my head I was. What the hell is with all the fake blocks!?
-
Metroid on the NES. I forget.. can you shoot UP in this one? I remember being horribly frustrated by the controls in this one, even after going back to play it on an emulator.
-
Legend of Zelda on the NES. I only ever played it on an emulator.. does the manual help any? I was playing it on 2nd quest mode and couldn't find the 4th dungeon.
-
Ikaruga is pretty hard and I only play on Medium. I've seen some youtube videos of absolutely disgusting 2d scrolling shooters.
-
The original Monkey Island. My friend and I are still stuck inside the Giant Monkey Skull.
-
Guitar Hero.
Laaame answer but there are some songs that require a lot of practice. I've beat Lou on Expert in GH3, and I've finished Jordan on Expert in GH2. Haven't spent more than an hour practicing the tapping intro to TTFAF, but it's all that holds me back from finishing it.
-
Miss Spell's Class. My best is 22 seconds =P
Just realized you don't need quotes around the url when you use (url=yadayada). Now I need to go back and edit like 3 posts I've made today. Also I'm giving up on the World's Hardest Game because it is getting reeeeaaaaalllly late. Also if you click some links around dictionary.com there's also Anagram Cracker. My time for the word 'Ignoble' was faster than 100% of the other players in the database. I haven't managed to crack the top 10 yet, though.