Honestly, I would say Ishiin from Sekiro, which gives me a hell of a trouble the first time I faced him.
Other good mentions would be the Orphan of Kos from Bloodborne, which is a nightmare and Manus from DkS1 but solely because he almost made me drop my Soul level 1 run.
The nameless king from DkS3 is also a good pick. Like, good lord that boss kicked my butt.
Are you kidding me?! I though about it while watching your Gwent commentary and was like "Nah, it can't be a referance to FFVIII, it's probably a coincidence since it's written without the E" but no it is. I was right all along lol