Oberyn takes this. I feel like he's in a league above where Brienne is sitting. She’s very good and very strong, but her best feats involve beating Loras Tyrell, who, while very naturally skilled, is very inexperienced and frankly doesn’t seem all that impressive when you really look at him. She also survived a fight against a half-starved, out-of-practice, and physically ruined Jaime Lannister, who had his arms and feet bound, hindering his movement.
In combat, she relies heavily on her superior strength and size to gain an edge, not unlike the Mountain himself. However, unlike the Mountain, she isn’t nearly as large or as strong. While she’s smarter and perhaps a more skilled fighter, overall, she isn’t as big a threat as Gregor. Oberyn proved to be more skilled than the Mountain in combat. He did get disarmed, but that was a failure of his equipment more than of him, given that the disarm was in the form of destroying his spear. If Oberyn can dance around the Mountain and cripple him, he should be able to do the same to someone of Brienne’s caliber.
I personally rank Oberyn alongside the likes of the Cleganes and other fighters who were skilled enough to gain national recognition and a cool nickname. This is a notch below the true legendary swordsmen like Jaime, Barristan, and Arthur Dayne. Brienne, however, is a notch below Oberyn, along with warriors who are highly skilled in their own right but not to a high enough extent to become especially famous for it. Of course, there are always exceptions, and people from outside Westeros are harder to place.