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!