He's not bad, but he's not one of the big ones. Spartacus, Gannicus, Crixus are the ones I'd consider "the best". Agron has always struck me as being second tier, with his Syrian boyfriend, the blonde German chick and some of the other named-but-no-one-really-knows-them slaves.
There hasn't been a Roman on the show yet who was even close to first tier combat ability. Spartacus, Gannicus, and Crixus really are practically "super men" when they fight. Remember the first episode with Spartacus charging into a horde of legionnaires? That's what I'm talking about. Considering Caesar is such a famous character I fully expected them to make him the Roman equivalent of Spartacus, Gannicus, and Crixus. The fact that he's more the Roman version of Agron is a lot more realistic and I like it.