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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I was going to answer Friede and Father Ariandel, but I'm playing through Bloodborne's DLC for the first time and I'd say Ludwig takes their place. I have a habit of checking my time played when I die a lot to a boss. I've spent around 8 hours on Ludwig so far. You know what really rubs it in? He's killing me with my favorite weapon, the weapon I specifically wanted to find in the DLC. That hurts.