Dracula from Castlevania. Took me way too many times. Cant think of any other.

My wife

Just for the sake of variety, I'll put my top 5:

1: Dracula from Castlevania III: Dracula's Curse

I've heard it isn't that difficult if you use one of the side-characters, but I played the whole game Trevor-only and I wanted to finish it like that. It was pretty tough, specially considering that they don't give you a checkpoint for continuing at the stairs before Dracula, like every other Castlevania does.

2: Diablo from Diablo II

This was pure pain. Did it solo on my first time and it was hard as hell. Dying again and again by that stupid lightning hose just to try to recover my belongings completely naked didn't help my morale either.

3: Ares from God of War

This one wasn't as difficult as some others in this list (definitely not easy though) but it gets a spot simply because of the part where you have to protect Kratos' family against Kratos clones. Doing that in God diffiiculty almost drove me nuts.

4: Aec' Letec from Baldur's Gate: Tales of the Sword Coast

Pure pain, incarnated. It took me a whole night of re-loading endlessly until I finally vanquisted this beast. The hardest part of this boss fight is that even if you kill the boss, he can take the body of one of the cultists like he is "The Thing".

5: Omega weapon from Final Fantasy VIII

This one I dreamt of beating when I was a child, but I could only stand against it when I replayed the game a few years ago. Even with my best team, the best magic equipped and a good strategy, I'd say that I only managed to beat it by pure luck. I didn't use the invulnerability items cause I wanted to make it as hard as possible.

Agree with OP Watchdog of the Old Lords was a bitch. That boss prevented me from getting the Platinum trophy for more than 2 months. Finally I broke and had to recruit two friends to help.

Dr. Wily in Mega Man 7.

I had a hard time in the witcher 3 with the baby you have to prevent the transformation.
When i was young i found difficult the final boss of infection on ps2

Can i say Gothic 2 Gold, every enemy is a boss.

I don't know what it is with that boss, either.  First time I played through Bloodborne, I got my ass handed to me dozens of times by her.  Hardest in the main game (her or Martyr Longarius).  

Then, second time through, beat her first try.  Haven't had difficulty with her since then. usually beat her first or second try, and have never used help or any form of Co Op against any bosses in that or Dark Souls III. 

I don't actually know of many super difficult bosses. I don't find most of the bosses in Bloodborne/Dark Souls all that hard, just a matter of not being an impatient twat. Take your time, pick your attacks, and don't get greedy. The enemies WILL open up for an attack.

That said, I agree with OP. Probably the hardest boss in that game was Defiled Watchdog, simply because you had half health, and a single mistake could result in your instant death. Plus, his attacks were super hard to dodge, he was hard to parry (if at all, I never was able to do it), and many of his attacks had additional flame damage.

The hardest boss I've fought recently, however, would have to be the Nameless King. Dark Souls III. In retrospect, really not THAT difficult, but when I was still learning how to fight him, I found he hit like a tank to the point where any one hit would take 2/3-3/4 of my health, had physical and magic damage, was hard as hell to see/fight, many of his attacks had weird delays that made them hard to dodge, he was flying around on a dragon so he didn't open up for attacks often, he sometimes did this fire-fall attack that, if it hit you, was an instant attack, and the arena was on a shifting cloud so I found it hard to judge how far away he was when I went to attack, resulting in MANY untimely wasted attacks.

And hat was just form 1. Once you kill his dragon you fight the Nameless King himself, which is even harder. He's faster, has a more varied attack pool, twice as much health, even trickier attack delays, and combos that mix up horizontal with vertical attacks, so dodging to the side or back isn't good enough unless you know exactly what variation of the combo he's doing. Hits even harder, doesn't give you time to heal, and has a massive health pool.

Probably the one boss that vexed me the most of the last six months. Even then it only took about 20 tries. Beat it the first day I attempted it. or was it day two? I think it was day two, I GOT to the nameless king at like 2AM, tried him once, said fuck it and tried again the next day.beat him after only a couple hours worth of attempts. .

I hated the Lazarevic fight on hard so much, that I didn't even play crushing difficulty afterwards.

Defiled Amygdala was hard and I needed a whole night, but that one actually was an enjoyable challenge.