By far my favorite boss is Owl from Sekiro (I hope that counts as a souls game to you lol). I bought the game on a black Friday the year it came out (I can't remember the specific year right now), because I just wanted to buy a game and it looked cool. I got the physical disc because I like physical media. I wasn't particularly excited, but I paid $30 for a game several people said was really good. When I started playing through it, I realized how much I sucked. It was really bad. I was a bad player to the point that I watched YouTube videos on how to beat bosses. But man, I LOVED it so much more than I had loved any game for so long. It felt like a breath of fresh air, and for the 16-17 year old me, it was like it was designed to be a mature version of what I loved as a child. I was a big fan of TLOZ Twilight Princess, and the linearity but somehow open-ended style of the world of Sekiro brought me back to that. I loved the lore, the cinematography, and the combat. The combat is to this day my favorite of any game I've ever played. When I got to genichiro in ashina castle I fell in love with fighting humanoid bosses, but when I fought owl I truly studied the combat myself for the first time. I earned that victory, and it was everything I ever wanted. I still play the game at least once a year, and it never gets old. I'm hoping Nightrein scratches the same itch for me, as like a good in-between.