It's not even close for me.
Isshin, the Sword Saint - Sekiro
This guy gave me fits for a solid 6 weeks. Firstly, he's got a troubling first part where you have to fight... something else. That fight isn't difficult, but if you're not paying attention it can take off vital health points.
Once you've got through that first stage the fight truly begins. And this is where I really struggled. I'm thinking I had 80+ attempts at the guy before...I sort of just did it.
And without wishing to make it sound like I had a cheesy moment like in a sports movie where it just clicked....it definitely just clicked.
There was a bit of commotion on the street, and my partner was running in and out telling me about it..but I'm totally zoned in. Before I know what is happening I've got the guy down to maybe 10% of his health and then the pressure hits you....
Thankfully I used my knowledge of these games to back off and play smart (I've been burnt by trying to rush in many a time before), caught his parry attacks and got the job done...
Not without almost missing the final execution prompt of course.
What about you? Who's the toughest boss you've ever faced?
Dunno if it's the hardest boss I've encountered, or even if it really counts as a boss, but the one that made me rage quit the game and not go back to it for several weeks was the first encounter with the Marauder from Doom Eternal.