while this is not my vote for toughest boss, I've never been able to beat Mike Tyson in Mike Tyson's Punch Out 007 373 5963 go try it (to this day, that code is still burned into my memory, just from writing it on a napkin to give to a friend at soccer practice when I was like 7 or 8)
If by tough, you mean a boss that requires an infinite amount of patience, skill, and luck to beat, then that's it. Videos on youtube make it look easy, but doing it yourself is quite different.
If you're lucky, you'll make it to the final form without taking damage; giving you enough total health to take down half of his while spending all your healing items before dying. If you're crazy, you'll try and try again without refilling on E-Tanks.
I dread hearing that boss theme now.
Alternate Choice: Akuma from Street Fighter 2 Turbo Round 100...FIGHT!
As gamers get older there's an expectation for games to mature with us, but the fact remains that simple fun with friends will never go out of style
Dr. Wily in Mega Man 7 Sigma in MMX1 and MMX3 In Dark Souls: O & S, the 4 kings (NG+), Manus, and Lord Gwyn if you can't parry that consistently which I am terrible at. In Demon Souls: Maneaters, Flamelurker, False King especially on NG+ Yiazmat in FFXII took me almost 8 hours straight to beat at level 85+ MGS2: The tengu soldier fights and metal gear rays on european extreme on a big boss rank playthrough. It's not really a boss but MGS1 snake variety level 1 vr mission is pure torture GH3: Da devil battle Chrono Cross: Dario DMC3: Vergil
All of these were some of the hardest bosses I have ever fought in my 20+ years of gaming. I'm not sure why so many people said the bosses in Ghosts n Goblins are the hardest. The bosses are easy but the levels are psychotic difficultly level, especially on NG+. I'm sure there are more I am forgetting that I might add later.
GT: TheAnalogKid06 Can't wait to play: Last of Us, Watch Dogs, GTA V, Killer is Dead, Dark Souls 2, A Link to the Past 2, MGS V, Amnesia: A Machine for Pigs.