I completed Call of Duty WaW at a friends place on Veteran moderately easily, all you have to do is just keep moving and keeping them back. KillZone 2 was difficult on Elite, so I'll leave it there. The AI is advanced and adapts to your every move, which I didn't think was possible so far.
Any platform I would have to say Diablo 3 on inferno or the original Metal Gear, but on PS3 I would have to say Resident Evil 6 on Professional was the hardest for me so far, although the new DmC Devil May Cry did add the Hell and Hell difficulty (you die in one hit, but enemies still have their same health from Son of Sparda) and that seems like it would be difficult to get through if you didn't unlock the Super Dante perk from Dante Must Die (since you can attempt Hell and Hell without playing Dante Must Die first)
star wars the force unleashed sith master difficulty...i know that everyone was stuck on kazdan paratus for 15-20 hours (10 minute boss on other difficulties)...thats the only reason why i cherish my star wars plat