What was the hardest boss you ever fought in a game?

The final boss of Van Helsing (1) was hard for me - took forever and had like 3 stages where he regened all his health, plus super strong attacks and minion summoning. Long battles just to fail are annoying.

There's several bosses in Sacred 3 that are hard just because they take so long. i remember some of them taking up my entire lunch break. most weren't terribly difficult, except i think once i had trouble staying awake during one battle...

The final boss of Dragon: The Bruce Lee Story (SNES, 1995 EU release).
The Phantom.
Fuck that thing.
For a little background: he was immortal, he was hard as hell to take down but when he had his HP at zero, you must perform a single specific move (the neck-tie) into nunchaku mode, which already was hard to figure out by ourselves.
No internet, no walkthroughs, I think it took me 10 years to defeat him.

A mech game called Slave Zero. The last boss was a gigantic robot, which was not specially difficult. The problem was that game has a bug at hard difficulty, in which in that stage some enemies are supposed to drop ammo but they don't. So you need to take the boss down with the ammo you enter the stage with. Considering the whole game is based around moving a lot and shooting at everything that moves without thinking about the ammo this represents a complete change in play style. I looked at walkthroughs and people mentioned this bug, but no solution or fix and this is a pretty old game. It took a lot of attemps, making sure every shot hit, because you have just enough to kill him with maximum ammo levels. But really just enough, a single laser shot gone wide means you will fail. It took me about twenty minutes each attemp, and had to try around a dozen times.
Also the final boss in YS III: the oath in Felghana. No bugs here, it is designed that way. I pretty much considered it impossible in my first attemps, it took me many, many, many tries before i took him down.

I'm going to add in that, thinking back now, King Leo from Kizuna Encounter was one that kept kicking my ass so HARD that it took me a year of trying to beat him once and ONLY once. And he was just the second to lass boss. Last boss was so SUPER easy that it made no sense. Fighting game logic, I guess.

Then there were the Four Kings in Dark Souls where, to this day, I STILL haven't been able to beat them. Dark Souls is the only one out of the three I have never beaten.

2007, fourteen year old me was trying to beat the third battle with Vergil from Devil May Cry 3, playing it on keyboard. I didn't find a better way, but to farm red orbs and buy around 40 holy water items. So, yeah, I spammed with holy water.

It's honestly probably something from either Dark Souls, or the Megaman series.
But the thing in most recent memory is the final boss of Ys I, or the optional boss from Ys VI: Ark of Napishtim the latter of which I still have only beaten in Boss Rush Mode because grinding to nearly max level in the actual game would take forever.

The first boss of DarkSouls 3.
SadButTrue.

Nameless King.
Though really that's just because the first phase is horribly designed bullshit where you're fighting the camera more than the boss. The second phase is great though. Its basically fume knight crossed with Ornstein.

Vergil from DMC5 is pretty hard too...and this is on Human. Royal Guard really helps against him.

https://www.youtube.com/watch?v=qBS6gTExHL4
Ancient Elchulus raid when it was just released. There was low level character & weapon cap. ~40-50min just to kill it if you were lucky enough not to die, plus all the break offs and stuff. It was simply amazing tbh.

Kil'jaeden from World of Warcraft. Tried to kill him with pug on HC.
After 28 wipes and hours of trying, we had to give up.

I made the mistake of putting mods without thinking about BG2. I do not know which one I got, maybe the Ascension, but in the "final battles" I died without successfully hitting even once. Finally I said to myself: "Hey, not all adventures have to have a beautiful ending, you're dead and bad guys win." There was also a dragon (I do not remember if in BG or BG2) that ignores you with a certain amulet, but if you attack him, he tears you apart in a blink ... Dragons in Baldur? I run away like a chicken

With everyone who desperated me most, and was not the final boss, he was a Russian in the Alpha Protocol. He used drugs, increased his speed, became immune to bullets and stabbed me mercilessly. Among the typical enemy reinforcements. About to leave the game as impossible... I loved that game, but I do not play it again for fear of that guy XD

Has to be Nightmare King Grimm from Hollow Knight.
So the normal fight was not that hard but neither was it easy. Turns out it had a second form (which is Nightmare King Grimm. I believe the moves were just faster and harder to dodge. It was so hard to complete it, but I defeated him by just learning the moves and having specific charms

View attached image.

Metal Gear Rising Revengeance Monsoon

I don't get hype for titan quest, I played it on hardest difficulty and was bored to death so I quit it about halfway.
Glad you made it though and had some good time with it even if boss was disappointment

I play nioh now from time to time, never ever had been dead so many times while fighting with bosses

I don't know that I've played that many hard games based on what I'm reading here. Then again if I have the ability to save progress or continue almost unlimited times I think I really don't consider a boss that hard. Now Arcade games where I'm pumping in absurd amounts of quarters I can remember doing that with Samurai Showdown (2? I don't remember that well), Dig Dug, and Die Hard.

I do remember feeling incredibly satisfied for beating a later boss on Mario Wii on my own since I didn't grow up playing on console, but I don't even remember which boss that was.

Most of my most frustrating gaming experiences don't involve bosses but things like jump puzzles or timed challenges. There's a jumping area in Half-Life that I gave up on several times either by leaving the game for weeks at a time or by cheating. Or there was one chase in Prince of Persia that involved nearly all jumping and wall running and I just could not get on particular jump.

Dark Souls' Gwyn when I did a Soul level 3 run. I don't know why he was the hardest boss when I normally beat him so easily, but I spent a few months to finally kill him like this.

First boss on Dark Souls, I don't think I can beat it lol

Camera. Be it a SoulsBorne game or Survival Horror (RE, EW).

Sol Gryn from I Wanna Be The Boshy . Pure cancer

The true ending boss from Cave Story, i think it's called Ballos. Not only the way to get to it is very cryptic and hard to survive, the boss itself is a bullet hell mixed with 2D platformer where everything kills you and you have to shoot a specific part of him, while surviving three phases. I managed to beat him only on easy mode after dozens of attempts.

I'm playing Titan Quest and at about half way through Ragnarök. Sometimes random boss enemies are more problematic than the final boss(es).

It's hard to say who was the hardest boss, I mean there was so many... But one that comes to mind is:
NME from Rise of the Triad (2013) - Shoots (smart) homing missiles, that can one shot you (two with armor). It took me a while to realise you can weaken it by shooting the crystals on the wall...

C'thun and 4 Horsemen from World of Warcraft classic. They were still ridiculously hard even after being nerfed. Noone on our server could kill them before the nerf and our server had some very serious raiding guilds including the one I was part of. Any other WoW boss regardless of expansion felt like a joke to me after dealing with those two.

Duriel from Diablo 2 gets an honourable mention.

wouldnt know right out of my head but the final boss in war in the north was annoying with his unblockable magic? attack.

Never finished Titan quest tho, I was co-oping it with a friend and we where in egypt, he was a summoner/healer combo and i was something with soul I think? We made a combo so we synergised with our summons. We strolled through the mobs like they wherent there. so we lost intrest in the game.

And to think of it we quit the game a couple of years before that because we got slaughtered in the begining. But we didnt know we had to spread our skill points over different skills early on, instead of maxing the ones we liked XD

Found later a guide that explained it was important to get as many skills early on (from the build you where going for) and later on, upgrade the skills themselfs.

Edit: The bosses in no more heroes where always a chore, but that can be because you get nearly no rest with the amount of times you need to recharge your sword with shaking the controler like a maniac.
At least it helped me save money on the gym or sport, got enough exercise from it lol

Every boss I fought in Boshy. And I only did the first three or so. That game is nuts.