Oenomaus! He was the only person to ever fight Theokeles 1-on-1 and live, and his training helped Spartacus and Crixus survive against him. The only time we know he ever lost lost a fight is when he was against Theokeles and after he had been tortured, starved, and injured. His skill and training made great warriors like Crixus and Spartacus, and he himself was a highly talented fighter with an affinity for all weapons and fighting styles. I personally think he was the best we ever saw.
After him I think it's
Spartacus
Gannicus
Crixus
Theokeles
Caburus
Nasir
Agron
Saxa
Naevia
Drenis
Ulpius
The rebel woman with the long black hair who we see in the background a lot in WoTD and goes with Crixus, Naevia, and Agron when they branch off
Crassus (as much as I hate to admit it XP)
Pollux
Also, considering that the Veteran is like an old man and yet he lived through the entire war and several years of being a gladiator, I imagine he's an excellent warrior as well, but we don't see him fighting much. Same with Leviticus, Litaviccus, and Lydon. Rabanus and Ortius were gladiators for a long time as well, and they lived through most of the war (Rabanus right up to the very end, if we know for sure that's him on the cross in that one shot).