You guys are all talking way new games. All the games in the past ten years are nowhere near as hard as the original games of NES. The newest boss I've faced that was actually so hard I couldn't beat him was the American Ultima on FFVII. I didn't realized this guy gets harder the higher level you are so I fought him as almost max level on all the characters. He has an attack that wipes out everyone on my team except Cloud with one hit and leaves Cloud with minimal life. Sephiroth's most powerful attack just always brought your characters to 10% life but couldn't kill them. You had to kill multiple body parts on this guy it took the combined forces all of three of my people to destroy them. Then on the next turn, THE VERY NEXT TURN, he'd just regenerate whatever body part I just destroyed. When I tried to revive my friends with Cloud he'd use that one-hit attack again and wipe out Cloud and my newly-revived comrade. Apparently the stratgedy was to fight him at level 60 or so but I didn't know he got so friggin powerful the higher level you got.
However the hardest boss I ever had to fight was the final boss on LoZ2. First off, you have to fight your way from the beginning temple across the entire danger-filed landscape to the last dungeon. Then you had to fight your way through the entire dungeon, which was relatively hard, so you could go fight Dark Link. Now this Dark Link way nothing like the Dark Link of OoT that you just had to know not to try to stab at him. This DL had every attack you had and was invincible to your magic, it was basically like fighting another player. If you made it past him you got to fight the last boss which was something like a giant phoenix that took most of the screen, spun around in an almost all-encompassing circle and shot tiny little kill bullets at you with, again, invincibility to magic. It might not sound hard but just try this out for yourself and you'll be able to see how ridiculous this boss was. Did I mention anytime you died, you didn't start at a save point close by, you started ALL THE WAY BACK AT THE BEGINNING TEMPLE.