My vote goes to Subaru from Re:Zero. I have never hated an anime character so viscerally.
Not only is he a useless waste of meat, an arrogant prick who refuses to ever learn or better himself in any way despite having the ultimate cheat to do so… He had Rem – the best best girl that ever was best girl – confess to him in possibly the most beautiful moment ever, and he’s like “that’s nice but I still love the boring cardboard elf chick who has been mostly ignoring me all this time.”
You would not believe how many people I usually get lining up to defend both the show and him.
I honestly had to stop watching before it finished because I just could not stand him.