Mike Tyson from Punch-Out was amazingly difficult. I've beaten him exactly one time, ever, and I could beat everyone else in that game with my eyes closed.
Sephiroth in Kingdom Hearts really wasn't that tough of a boss. He had one incredibly cheap attack that stole all of your HP and MP, but once you knew it was coming (and had an elixir queued up) he certainly wasn't unbeatable. I don't understand that inclusion on the list.
I agree that RPGs are somewhat questionable for inclusion here, but if you're going to put them in, at least pick the right ones! The final boss from Dragon Quest/Warrior 2 was incredibly difficult (Malroth). You had to go through a huge castle just to face him, and then he would casually kill you out of hand 9 times out of 10. No saving or getting your HP back right before a boss fight back in the 1980s! Some of the optional battles in Final Fantasy V were also among the toughest I've ever seen. I don't know if they were changed in the remakes or not... that was one of the few RPGs I genuinely could not defeat certain foes.
Any of the Ninja Gaiden final bosses would also fit well on this list. I could never beat the final one in NG2 without dying... Some of the old Fire Emblem games were also just ridiculous. There were a couple of bosses in Fire Emblem 2 (Fire Emblem Gaiden) that I simply could not beat, no matter how I tried. Conversely, none of the Mega Man games were that hard, even the first one - although that was the closest.
