Elementalkin
Altessa Belmont
Not going to lie, I'm a sucker for the stealthy "kill-you-in-your-sleep" type. June has the stealth and reflexes that Korra has yet to develop. Also, since June is such a good tracker, she could easily tail Korra, watch her train, and pick up on her flaws without being noticed.
Well hypothetically this would be an abrupt duel, not something either could extensively train or prepare for. And are you sure about Korra's reflexes? I'd say she demonstrated that fairly well against Tarrlok.
My apologies, missed the part about the abruptness of the situation. She did do well against Tarrlock, but I don't see her doing nearly as well against June, who trained her whole life to be stealthy. Korra is much too brash and ready to hop in head-first.