Let's see...
1) Bronn.
a capable fighter, smart, fights using any advantage he can get because he knows his limitations... but indeed, he has limitations. In the books, he simply refuses to go against the Mountain because he knew he would have been toast, no matter how many gold or noble titles Tyrion could offer him.
So, he's strong and smart, but not the top dog between the possible choices.
2) Brienne
She's a competent fighter, but she's not in the same league of some other ones.
3) The Hound
He's got the same positive qualities of Bronn, and probably he's even stronger. High pain treshold, which helps a lot. A good candidate.
4) Loras Tyrell
Amazing tournament fighter, but i don't know if i'll trust him on a dirty fight with no rules. I'll pass.
5) The Mountain
this one is a beast. his only disadvantage is that he's slow, but this could be mitigated by the fact that i would pick him in the "zombie version". No chance to slow him down, to hurt him of fatigue him. A relentless monster, you'd really need to literally cut him piece by piece. A very solid pick, in an arena fight for now he's my first choice. Let's see who's next...
6) jamie
eh, with both hands he's a hell of a fighter. He's among the most skilled swordsmen in westeros, but he himself acknowledges that there are some other ones stronger than him. A good pick, but i don't know if it can beat Sandor.
7) Barristan Selmy
if only he would be younger...
8) Oberyn Martell
Oh, i like him... fast, very skilled, uses poisoned weapons, got a greater reach with the lance.
Alas, we saw him fight only once, in a circumstance where he was clouded by vengeance and the will to play cat and mouse. Could be able to kill the majority of the opponents in this list and also many other ones... but can he surpass the mountain?
9) jorah Mormont
i would put him around the same league of Bronn, maybe slightly lower. It's a no.
10) Khal Drogo
truly amazing fighter, but i fear the lack of armor can be his doom in a challenge like this one. I'll pass.
11) Roberth Baratheon
eh? no.
12) Syrio Forel
i love this guy. he's skilled as much as anyone else, if not more. He probably can judge his opponents even better than Bronn, and was able to hold back armored opponents armed with a training stick... pretty sure he's a deadly opponent, he would be surely a good pick, i'd put him an a level even superior to Oberyn.
so, my candidates are ATM (and in this order):
1) Syrio
2) Undead Mountain
3) Oberyn
(with the Mountain and Syrio very closed)