For me, it remains Sephiroth from the first Kingdom Hearts game. I've beaten gigantic creatures, Weapons, big ships that go pew-pew in every direction, things I can't injure, things that don't die like they should when they should, but this particular Man in Black is truly a monster, IMO.
There is something ridiculously intimidating about his movements and the reach of his weapon. I remember that he had four or five attack patterns depending on the level of his health; I really disliked how the game dangled hope by finally letting his HP bar go down after hitting him a bunch of times, then changes his attack pattern to completely throw your timing off.
Of course, I know HOW to beat him - like most bosses, it's about adapting to the pattern, memorizing the motions, and eventually relying on instinct and muscle memory to get you through it - but lining everything up without screwing it up has eluded me so far. I have never spent as much time trying to overcome a boss as I have on this guy.
I have to admit the radar noise does break the immersion. It could have been a lot more subtle and still usable.
Mailed my left joy-con in for repairs today. Hopefully they can get it back to me quickly, before I go into Zelda withdrawal. I had to save and quit at an awful point--I just found the Forgotten Temple and enjoyed the pleasant surprise you get once you walk fully inside. Spent entirely too many of my final hours trying and failing to farm horn fragments--getting close enough and then getting the shot exactly right is pretty tough, and time-consuming.