My favorite is the twin princes. It wasnt crazy hard or anything but its the only boss fight i thought was genuinely fun. I was glass cannon pyro at time so i couldnt afford to be hit by anything. In turn i mastered its moveset pretty easily so it was nice to dodge a boss perfectly to victory. Plus the music is great. Im very glad they chose to extend it with the revive mechanic. I would actually let myself die at times just to play it again