For me, (even though i never actually killed him) i have to say Midir. I tried beating him a few times and i just gave up afterwards.
A boss that i found actually quite easy would have to be the dancer of boreal valley. I killed her after about 2 tries. I also killed her surprisingly quick with most of my estus left.
Everyone has different play styles when it comes to Dark Souls/Bloodborne, so a boss you may find easy might be hard for someone else. Leave your bosses below!
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Havent played through all of blood borne yet, but it definitely has harder bosses than any of the normal dark souls games. At least in my opinion. Darj souls has some pretty difficult and annoying bosses, however i found many blood borne bosses to be infuriating at times.