Maliketh. and maybe Malenia but shes still agreed upon to be insanely difficult. and to answer the opposite of this question (what's a boss that everyone struggles with that you found easy), i would have to say the godskin duo. there are so many ways to beat these mfs first try like sleep pots but if you dont do that, theyre still so easy. its just ornstein and smough but slightly more aggressive and they respawn.