At first I thought to place, instead of Gannicus, the famous Undefeated Gaul, Crixus, but he dissapointed me a little during the current events in Vengeance (let's face it - the Undefeated Gaul, a very well-trained gladiator, a former champion of Capua, lost a fight to a common killer and a mercenary). Gannicus has something in him that draws a viewer to him. As Crixus and Spartacus he was a champion of Capua, and he stands high above many other gladiators. And, as for now, only Spartacus can be matched with him when it comes to fight.
Notes after the season finale:
I think he is the one male character, who evolved the most during this season. From a simple alcohol and women addicted man he became a co-leader of Spartacus army. He lost his friend, and he killed the man who was responsible for his death.