Honestly none of the bosses are hard for me anymore except for Moon Lord, and he's not all that hard either. But out of all of them, he's the only one that I can't beat easily once I learn his pattern. With every other boss, you learn how they work, and they're cake. With Moon Lord, learning how he works makes him easier, but it doesn't ensure your victory.
I voted for Duke Fishron because Moon Lord wasn't there and Duke Fishron is the boss who's killed me the most besides Moon Lord. But really Moon Lord is the only correct answer.