The term “hardest boss fight” is used far too commonly. However, I have found one that takes the cake.
It started when I was watching Saltybet, and noticed (repeatedly), a character called “General”. Looking up the origins of the general, I found that he was from a game called Kaiser Knuckle (known as Global Champion in a small US release), an unsuccessful and otherwise unspectacular Street Fighter knockoff.
Stylistically, the General is nothing but a cheap imitation of the legendary M. Bison, but in gameplay terms is so hard that it took twenty years for proof of a legitimate victory to be posted.
This is the proof of the victory over the General.
The boss got so hard that the developers nerfed him in a subsequent (and never actually released) update to the game.
The General is a relic of an era of less-strict game design-only stronger to his cheap-boss compatriots in relative terms, but overwhelmingly stronger nonetheless. Whether due to rushing the game or just horrible quality control, what should have been dialed back in testing ended up as this footnote in history.
Sometimes a limit can be reached, and the General was apparently that limit.
One final note: It’s a sign of MUGEN munchkin arms races that the General is not on the absolute highest tier in Saltybet.