For me, it was the Ogdaron.
I just got done playing with a bunch of friends, and I wanted to do one more mission (12:30AM) and just explore the Rotten Vale. So, I start up an expedition and go into the depths. I only had 8 Potions, and 3 High Potions. Suddenly, my health starts slowly dropping and the handler states “The air is poisonous”, and I decide to GTFO.
As I head up the hill, this large, dog-like creature that is skinless, turns the corner and stares at me. I stop in the bushes and forget to crouch and hide, and suddenly it is on me. This creature is viciously swiping and jumping and all I can do is avoid its attacks, while choking on poisonous gas. Oh, and he connects for the first time and puts a bleed on me that causes me to take damage when moving…
I hid in the bushes, bleeding and poisoned and I am already down to four potions left. During this struggle I was able to get in a few hits and dismember its tail. We are both wounded, and the beast finally leaves to give me a breather. I should have gone back to my tent, stocked up and went after him, but I was chasing that experience.
I top off and find the monster waiting around the corner, stupid hunter. We battle again, and it just feels like a bloody brawl, and every hit just feels sickening with the crunch of steel meeting flesh or claw rending mine.
Finally, the beast runs away, and we are both on the ropes. One potion left, and at 40% health. I chase after the beast, and the bloody dance starts again. Finally, it ends with me landing a lucky jump and the beast is slain, and I sit there with my health ticking down due to a bleed. I crouch, take the last potion to heal up, and then proceed to carve my prize.
Since that fight, I can’t put the game down. I haven’t felt this into a game since Dark Souls and Bloodborne. That rush is amazing, and I want more.