man, I don't know where to begin with this one
Mike Tyson (Punch-Out)
Sans (Undertale)
Whitney (Pokemon GSC)
Rugal Bernstein (King of Fighters '94)
Tyson and Sans I don't think need much explaining. Whtiney (well more specifically her Miltank, I would vote for just her Miltank if I could) is one of the only bosses I can think of that some (not me.) think of as so hard that many memes have been produced about it
I put Rugal up there because SNK Boss is actual video game slang and I'd wager if you had to pick only one, it has to be him or Geese.
shout to MM7 Wily though. that shit is unbelievable I can't believe I didn't remember that one.
Born to lose, live to win!