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Unironically Lies of P: Overture. The final boss of that DLC was a transcendent experience.

Arlechinno is possibly the most fun I've had with any boss in any game ever. Took me probably about 5 hours to beat him and at no point during those 5 hours did I get angry or feel like the boss was unfair bullshit. It probably helped that I deliberately didn't help Lea up to help me, so I couldn't really get mad when the game was offering me help that I wasn't taking. When I finally clutched out the win my heart was pound in my chest, I was breathing heavily and my hands were shaking to the point I could barely hold on to my controller, genuinely an all time memorable gaming experience, put any boss FromSoft ever made to shame.