Clearly it's Vargas.
Terra Branford
Locke Cole
Celes Chere
Edgar Roni Figaro
Sabin Rene Figaro
Cyan Garamonde
Setzer Gabbiani
Gau
Shadow
Relm Arrowny
Strago Magus
Mog
Umaro
Gogo
Interceptor
Kefka Palazzo
Emperor Gestahl
Leo Cristophe
Banon
Ultros
Clearly it's Vargas.
I'd say either nobody or everybody. It's an ensemble with our core group collectively forming the protagonist. At this point, I really don't think there's any reason to single out any one character that has more plot importance because the game has been designed to just not have a single protagonist.
Even though Terra is an important character plotwise, without Celes the party would be nothing, how would they get an airship for example?
I don't remember Terra doing something very important for the returners' cause. Please enlighten me.