Although, I thought Demon Souls was pretty hard when it came to boss fights, but it was because I didn't bother to stick around with that game.
"I've begun my voyage in a paper boat without a bottom; I will fly to the moon in it."
"When this paper aeroplane leaves the cliff edge, and carves parallel vapour trails in the dark, we will come together."
I've heard Dark Souls is pretty hard, I never played it though. Devil May Cry 3 was pretty hard too, especially since the hard difficulty in the japanese version is the normal difficulty in the North American and European versions.
I've beaten a lot of games but out of all the games I've played I would have to say the final boss of Metroid fusion is a pain in the ass and Miss pac man.