So many RPG bosses... I show something from other side.
I'm suprised that nobody mentioned any Metal Slug game. Bosses were epitaphiums to horribly hard levels, where every soldier could kill you with one hit. And they have almost always have backup - tanks, airplanes, whatever you want. You can today easily pass it on emulators, but on those days you would spend fortune on it. Try to pass this game on one continue, you will quickly understand.
Serious Sam First and Second Encounter is also pretty impassable on harder mods. Hard mode is impassable for most players. Serious mode is barrier for veterans. Mental mode is something out of reach for normal men.
Every average NES game was harder than average game today. I remember Werewolf: the Last Warrior, where I had big problems with boss of second level. Third level was out of reach. Never pass it. Anyone played Battletoads - not heard, but played? Level 3 was barrier for most players.
But the hardest thing I had ever meet was Xaero on Nightmare level from Quake III. He's not really boss, but bot, which has same points of health as you, and same weapons. Differences start, when you try to hunt him down.
You have been spawned. You are casually walking towards weapon, and got shot from nowhere. You hide behind column.
And it's over. Whatever you will do, you are nailed down. Because he's waiting in safe place when you will appear again, and is aiming on column. You're alive, but you have already lost first frag. You can, at best, try to hit him just once. Let us agree that some miracle had happened, and you had hit him before death.
You have been spawned again. And hit. You are falling from arena.
Spawned again. Grabbed rail, and maybe hit him once. Too bad. He already healed up himself. You lose.
When I won with him, I was feeling like I had already aquired everything that I wanted in gaming. Something like ,,I can die now".