Which boss fight did you find most challenging, and why?

I miss playing Sekiro but at the same time I'm grinding out my backlogs plus I'm low on time cause I'll soon be going to do my degree either ways I do keep getting "flashbacks" of Sekiro so yeah I'm on here reminiscing. I do feel like the mini bosses kicked my butt a lot more than the actual bosses did but at the same time the mini bosses are what actually trained me in the game to fight the bosses, i really tried hard to learn the parry system through fighting them.

All the Second Bull sucks. I don't care if you think it's great but it's bs