I thought it was established already that there is no best, and that it's impossible to rank the best warriors. All the great warriors/swordsmen in the series fall within the "margin of Selmy".
A warrior without peer
those are fine words, Your Grace, but words win no battles.
[...]
He did, ser, but
I have seen a hundred tournaments and more wars than I would wish, and however strong or fast or skilled a knight may be, there are others who can match him. A man will win one tourney, and fall quickly in the next. A slick spot in the grass may mean defeat, or what you ate for supper the night before. A change in the wind may bring the gift of victory.
The only one I really feel stands above all others in pure swordsmanship is Syrio Forel, however based on what we've seen thus far he's at a great technological disadvantage against Westerosi knights, since he doesn't wear armor or a shield. That may change in the future though, if the bravos fighting style is explored more. We see bravos in Essos, and one is even second-in-command in Brown Ben's sellsword company, so obviously they have some way of competing with armored men.
That being said, at the start of the series the top swordsmen are Jaime, Bronn, Sandor, Gregor and Barristan. Qhorin Halfhand or Mance, or maybe both, may belong there as well.
Based on hype Oberyn Martell and Lyn Corbray are also probably top swordsmen, though we haven't actually seen it yet. Brynden Tully as well. Another guy I get the distinct impression is very dangerous is Stannis' second-in-command Richard Horpe.
I really don't think Loras and Garlan Tyrell make the cut. Loras is known for being an excellent jouster, not a swordsman. And the only thing we know of Garlan is that he's better than Loras and that he trains against 3 or 4 guys. One handed Jaime also trained with Ilyn Payne, and we know how well that went.
There's a difference between
sparring against 3 random fodder with blunted weapons in the practice yard and beating 3 determined, skilled, fully armored knights who have the intention of killing you. Almost anyone can do the former, while the latter is impossible to achieve.