i gonna have to say bowser from mario 64, i beat him once after many deaths, couldn't do it again after that.
the final boss of sonic 3, i didn't do this one alone, my bro was playing as tails. it was my job to dodge the attacks which included the giant lazer beam, it took me many failed attempts before i could time the lazer jump just right. my bros job was to attack eggman as often as he could, let me tell you he missed a ton of attacks.
1st phase was no problem, it was second phase where it got nasty.
andross in starfox adventures, alot of people say this was easy i say bull, i can't remember how many days or weeks or maybe it was months that i tried and failed.
riku replica 4th fight on chain of memories gba version, i got so fed up i threw the game aside for several years and forgot why i had put it down when i picked it back up.
kesseler on infamous, if i wasn't fueled by rage and adrenaline i think he would have been harder.
final boss of portal 2, only because i had no idea where to stick the cores in him first time around. didn't help in the least that the whole part before that had me scared s**tless and i was having a hard time thinking clearly. when a boss can use an entire building [that you are walking around in] to kill you it gets nerve racking.
if we are counting bosses that we are stuck on as well, then the final boss of drakengard if i was allowed to fight on the dragon like i could in every other part of this game, i would have had no problem. but it's a damn reaction\memory game, [i still think wtf!! !] i've been stuck on him for 8 years i think now, i haven't even gone back and tried to beat it since i got stuck. not even codes and cheats help here.