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?
I thought returning from Duna was much harder, because there was so little room for error in terms of resource usage. It might seem like you planned everything out perfectly, that you were within your error tolerance (but not ideal) for fuel consumption and trajectory, only to find that you only have the exact bare minimum of fuel required to return. And since no launch or transfer ever goes perfectly, you'd be stuck in an orbit that got you close to Kerbin, completely out of fuel and no way to slow down and stay within Kerbin's SOI or avoid burning up in reentry.
Time to revert to the VAB to try and eke out a few more m/s of delta V, and try again...
Edit: and I never actually made it back from the surface of any other planet/moon that has an atmosphere