TV
Brienne and Oberyn are absolutely not second-tier and Jaqen would probably be top tier too if based on presentation rather than feats.
Oberyn is very, very good at dueling with his short spear and light armor. With his reach advantage and superior skill and agility, he was able to dance circles around the Mountain and slowly dissect him almost at leisure. But he doesn't have leisure time here. He's in a big meelee with threats coming from multiple angles at once. I'm actually iffy on whether he beats Brienne one on one; he was absolutely the best character to match up against the Mountain, and would have had a a harder time against people almost as strong but way faster and more skilled like Brienne or the Hound.
In a grand meelee, Brienne definitely has the edge over Oberyn. Valyrian steel blade, great reach, heavy armor, high damage soak and strength enough to make piling on her a difficult prospect. That said, those same qualities make her very liable to draw aggro from multiple sources.
Therefore, I'm backing
Bronn. Bronn isn't the fastest, the strongest or even the most skilled fighter here, but he's got sufficient quality in all of them to remain competitive. More importantly, he's smart, full of little tricks and has excellent battlefield awareness. He's a mercenary who fights knowing that the coin he's earning is only good to him if he's alive to spend it.
https://www.youtube.com/watch?v=jzNn7oF6eEY
We've seen Bronn enter chaotic situations time and time again, and he always manages to get an edge, always manages to avoid taking anything more than superficial damage. He has the best risk assessment of the fighters here. For these reasons, I'm betting on him.