Hmmm... tough question.
My first thought would be Barthandelus because he's a Fal'Cie. However, Caius is immortal due to the Heart of Chaos. Even in the ending, the Heart of Chaos is destroyed and Etro dies, but Caius is shown to have survived. I'm not entirely sure, but if I understand correctly, even though Etro dies, this merely means that Caius turns mortal. So, in that aspect, I think Caius would win, because despite being a Fal'Cie, Barthandelus is still capable of being destroyed. -- ∙ ∙ 04:05, July 31, 2012 (UTC)
That and Caius can turn into Chaos Bahamut and even Jet Bahamut!