I meant before he lost his hand (now he isn't even one of the best of his time, let alone one of the best ever). We have heard that he is good, has trained and while he became the youngest ever member of Kingsguard that was purely for political reasons (Mad King wanted the son of Tywin near him almost like a hostage). We haven't seen what he has done and probably his biggest challenges were fighting Eddard Stark (who while a great figher wasn't near the best one) for a minute when Jaime although totally dominated the fight, still couldn't defeat him and cutting a few Stark men while they were under orders to capture - not kill - him. I also don't remember him winning any significant tourney so personally I would put him on the second level of fighers together with both Clegane brothers, Greatjon and possibly Ned and Overyn. I think that in their prime Robert, Rhaegar, Barristan, Arthur and The Smiling Knight would have defeated him.
Saying that, he might have been the best figter during the events of the story (with Rheagar, Dwayne and the Smiling Knight dead while Robert was fat and Barristan a bit old).