I tried to post a video and it won't show up, and plus, I can't even delete this stupid post...

Kalameet from Dark Souls was particularly nasty. Trying to tank his attacks is not a great ideas since his attacks will either break your poise or get close to, so you need to remain as mobile as possible. Unlike other bosses, he doesn't really have any clear openings when you can run up and swing at him so you just need to hit him whenever you can.

Death from Castlevania 1 without subweapons is also very difficult. His attacks are random and they hit hard, on top of this Death himself also moves around and he can trap you into a corner where he'll just kill you from contact damage. If that wasn't enough, the corridor before him is full of annoying enemies which you need to get past every time you want to try again.

Gid of War 3 that freaking three headed dog of Hades after defeating that Ice scorpion. On the hardest difficulty setting that is and Rom the thousand eyed spider alone (as a true hunter and yes I died more times than I would like to admit) on new game+

The first time a flamethrower was actually useful ;)

I wouldn't be able to name a single one as the "hardest". The first one that came to my mind when I read the title, however, is Metal Gear Rising's final boss. Considering how that game gets progressively easy as you keep going, the endgame boss was such an unbeatable foe. It literally forces you to reassimilate your gameplay style, all in one combat, to find a chance to win.

Dark Aeons in FFX. Final boss in Heavenly Sword, I gave up and didn't care.

Post-game Kingdom Hearts bosses are impossible. People complain about stuff like Dark Souls but it's nothing compared to the difficulty of some optional Kingdom Hearts bosses. And I'm not even talking about Proud Mode.

Of all the bosses I fought in Bloodborne.  Micolash, Host of the Nightmare and Father Gascoigne Werewolf always given me difficulities, especially in New Game + 2...  I tried embeding my actual videos, but they're not showing up so I'll just post the links instead...

Bloodborne: Micolash, Host of the Nightmare Boss Battle (NG+1)
https://youtu.be/tNzU6SlDxsA

Bloodborne: Father Gascoigne Boss Battle (NG+2)
https://youtu.be/pftmAM-J60E

mine from yakuza 3
https://youtu.be/0ti8tKjuByo
N-Gin crash Bandicoot 3
https://youtu.be/kncIep-gGyw

I have.  Nobody is a worthy adversary.  So I had to fight myself.  Sometimes I even get a little fright when I look in the mirror.  I'm an absolute animal!!