A couple of selections of easiest and hardest:
For the top 5 hardest games i've played: (In no particular order)
Donkey Kong: I've never gotten past the first level, which to some gamers is a real social faux par, a sign that I might not be a true gamer... But fuck that.
System Shock 2: The impossible difficulty is exactly as prescribed. A lack of ammo, items, insane enemies made me abuse quickload and quicksave like my john thomas on a lonely saturday night. (much like this one)
Guitar Hero III: The final set of songs on expert are fucking impossible, and this is strange from someone who has managed to 5 star the previous titles. (I'm not particularly proud of that, when that time could have been use playing my real guitar and actually being able to play reigning blood)
Wipeout 2097, Wip3out, and Wipeout Pure: Whilst on the lower levels, wipeou seems rather placid, phantom class has reamed me over and over again, only to have me crawl back for more. I cannot move things that quickly without some rather expensive and illegal substances.
N & N+: On the free PC version of this there is over 300 levels. I have only reached level 22, and this game threatens my sanity at every turn.
My top 5 easiest games:
Gears of War (Co-Op): Oh yes, there were points where this game was kicking both my friend and my own asses at the same time with steel toed shoes, but for all intensive purposes, was a very easy game, especially compared with the FUCKING HORRIBLE controls of most console shooters. The amount of times I have had my ass handed to me by consolers at halo is insane, yet as soon as it's PC time, victory is mine. But Gears of War, piss easy.
Max Payne 2: Brilliant game, but far too easy. a 10 completion time isn't unheard of.
Need For Speed Underground 2: A horribly sub-par series which a retarded monkey could finish, needlessly extened with no real sense of acheivement in the end.
Wipeout Fusion: Reducing the previously brilliant formula of the previous wipeout games to nothing, and stripping every bit of skill out and replacing it with weapons was not a wise decision. Every race can be won with rockets, or a plasma bolt. No skill at all required.
There's probably a lot more I've missed, but my phone is rining with a possible booty call.
Damn, just another telemarketer.