That he's a good commander?I'll give you that....but the fact that garlan was chosen to lead the attack on Stannis says a lot.
Also, I'm assuming that when discussing the best swordsmen we're referring to duels on foot. A knight's role in battle is heavy cavalry. They fight armored, on horseback, and with a lance. Duels and large scale battles don't really overlap that much. A great knight might not necesarily be a great duelist, and vice-versa.
Bronn is at the very top. He's stated by Tyrion to be almost as good as Jaime with a sword. And his feat of defeating a fully trained and armored knight while he himself was fighting unarmored is as of yet unrivaled in the series, with the exception of Oberyn Martell, who fought with a poisoned spear.Bronn HAS to be at least top 15 to have survived this long he might have the most battles we actually read instead of just hear about.
edit:
I'm not really sure Garlan even led the attack on Stannis. We know he was on the front line because he wore Renly's green armor to scare the shyt out of Stannis' troops, but I'm not sure if he actually led the attack.