I think before we can answer this question, we need to define a metric for "good swordsman". Most people are implicitly suggesting a head-to-head tournament in a knockout or league format. I the format of the tournament is important. In a knockout tournament, I think there are some rock-paper-scissors stuff here that might affect who wins overall. The Mountain might kill someone like Jaime Lannister, who, while possibly the most skilled of all, maybe lacks the agility and quick action to defeat such a powerful opponent. OTOH, Jaime might defeat Oberyn, as Oberyn's twinblade/spear may be much less effective against someone as balanced and skilled as Jaime. Imagine if Bronn took part too (for the prize money, presumably) - he would surely do well against some but not all of his opponents. Expect plenty of upsets!
A league format, in which everyone plays everyone, will probably be won by Jaime or Oberyn, as it tends to select for the best overall player and completely eliminates rock-paper-scissors. Selmy is -- or rather was -- one of the best swordsmen in Westeros, but he's old and will probably be disadvantaged by the format: it's a much longer competition than a knockout and I think fatigue will set in after a few days of fighting.
There are of course other tournament types, like the ones used in
Chess and TGCs. These tend to result in the best overall player winning, and minimise (but not eliminate) rock-paper-scissors. However, upsets are still possible. Again, I'd expect Jaime or Oberyn, who are supposed to be the best overall fighters.
However, a tournament isn't the only metric. Rather than competing head to head, one can judge ability by how well they do at a particular task. For example, following The Hound's advice, we can imagine how many Meryn Trants it would take to kill each fighter. According to The Hound, any boy with a sword can kill 3 Meryn Trants, so we already have some sort of comparison. However, Meryn Trant and a couple other swordsmen beat "the best swordsman in Braavos" (the waterdancer bloke), so perhaps The Hound's opinion of Meryn Trant isn't quite objective.
Anyway, in units of Meryn $%^&ing Trants here is my full answer:
[TABLE="head"]Swordsman | MFTs
Ned Stark | 8
Jaime Lannister | 16
The Hound | 12
The Mountain | 14
Barristan Selmy | 13
Jon Snow | 4
Oberyn Martell | 15
Brienne of Tarth | 10
Stannis Baratheon | 9[/TABLE]
I suppose he might have taken a vow not to use again a real sword or something. Is the scene having the same ending in the books as well?
