Obviously most fans want to feel like their group is victimized for victimization perks, but objectively speaking there are always those groups which receive more hate than others.
There's always casual hate, but other times there's very extreme and consistent hate. I'm not counting China because Tzuyu/Twice would win by default, as there's billions of people there.
I think it would have to be BTS for boy groups, they definitely stepped on a lot of big fandom toes and the hate for them beating bigger groups at success lingered for many years until they became undisputed #1.
For girl groups, there's a lot of incidents but very few cases of continual hatred, so I'd say GIDLE because they still get hated on for ethnic hip and in Korea the Soojin thing is still unresolved.