Well excluding the DLC it’d probably have to be Pontiff Sulyvahn for reasons I’ve gone into earlier in this very thread.
There are a few bosses that share those qualities and comes a close shared second place, the reason I think Sulyvahn gets the top spot is because he was the first boss that really clicked for me. Up until that fight I didn’t feel like I was really conquering the fights, I didn’t feel like I was getting gradually better until I finally had the skill to match the challenge.
Bosses before Sulyvahn I mostly felt like I either got lucky or beat on a fluke or they were just easy encounters that didn’t really require much learning and improving.
On an afterthought the Abyss watchers could have been a favourite as well, didn’t feel like it at the time though. It’s a few months ago now so I do not fully recall what I was thinking at the time. Sulyvahn stood out and for some reason his fight is where I started to really properly enjoy the game.
The Nameless King is a great fight too, much for the same reasons, but he had been a little hyped up for me and I felt a little disappointed when it ONLY took me 3 days to beat him. Prince of Lothric could also have been a contender but that fight was far too easy and over much too quickly unfortunately.
Finally I must stray from your restrictions to say Ashes of Ariandel final boss offered me a very similar experience to that of Sulyvahn with a few extra flairs that edges him out to get on top for the full game.
GET THE DLCs, it is SO worth it!