I'm going to go with Korra on this one Korra in book 4 the finale shown how much of a powerful bender she is (she rarely used the avatar state) comparing book 2/3 Korra to book 4 there is a big difference the most recent fight we got from the twins was when they fought the red lotus members and they lost (even with Zuko there)
Idk to me Korra seems the clear winner
I'm leaning towards Kuvira for some reason because her fighting style seems to be the kind that Toph has even though it doesn't show that well in the show since its pretty fast paste. What I mean is Kuvira tends to be more of a counter fighter manipulate her surroundings to her advantage and you see her do it often vs Korra the first time they fought vs Suyin etc like a master earthbender she waits then strikes which I believe to be a bit problem matic for Katara sure they may be surrounded by oceans BUT as staded the ocean is down meaning Katara will need to focus more to get to the water and need to be able to avoid Kuvira's quick attack yes for an earthbender she is pretty fast. Waterbenders like Katara have the reflexes to strike back as well but since the way Kuvira seems to fight I feel like taking her side on this one.