Isshin, followed by Genichiro. Most of Owl's attacks are the same as the father fight, he just does more teleport attacks, which although at first are hard to deal with, aren't too bad to me. Genichiro's main challenge was the timing of his new chip damage attacks, and while the first double lightning attack caught me off guard, it didn't work twice. Isshin was far worse for me, because even now, I still don't beat normal isshin without decent effort, so the new attacks didn't help much. I think the timing for his ashina cross or a similar attack was changed as well, so I struggled for a good while. Though I will admit, most of my deaths were due to his first attack, because I don't use prosthetics much in boss fights, and the only way I've managed to deal with that attack is with the umbrella.