20DEFEAT: THOMAS MARVOLO RIDDLE
Let's get the most obvious out of the way first: the one and only Lord Voldemort, otherwise known as Thomas Marvolo Riddle (just don't call him that to his face). Bellatrix was his right-hand woman and when she was finally taken down, he bellowed out in rage at her passing.
was certainly a powerful witch who earned Voldemort's trust, it's highly unlikely she could have defeated him in combat. His very presence was intimidating and awe-inspiring to her, which would be enough to put her at unease and make her an easy target for him to take advantage of in a duel.