Its obviously Ventus in my opinion, since look at the facts:

 

Terra ended up as a vessel for Xehanort due to using the Darkness to fight him in the keyblade graveyard, and thus made things even WORSE, since now Xehanort had achieved his goal of finding a new body.

 

Aqua tried to free Terra from Xehanort's control, but when he fell into the Realm of Darkness and Aqua saved him but got herself trapped in the process, she made it impossible to Save Ventus since she's the only one who could find him, and she saved Xehanort who created Ansem and Xemnas and Org. XIII.

 

Ventus however, caused 2 positive outcomes:

 

His heart slept with Sora, which made the Keyblade see him worthy and take him after Riku turned to the darkness.(Sora however got the power to wield the keyblade by himself when he stated that "My friends are my power!" And now he has his own)

 

And the fact that he DESTROYED THE X-BLADE, and stopped that portion of Xehanort's plan. So he did something useful to stop Xehanort, unlike Aqua and Terra who inevitably made things worse by their Rash Actions.

They'll help, but ultimately, i think Sora, Riku and Kairi will finish the work for the most part.

Who knows who will finish him off? I think it would be proper if Ventus, Terra, and Aqua give Master Xehanort the final blow since they left unfinished business in the Keyblade Graveyard.

Who knows who will finish him off? I think it would be proper if Ventus, Terra, and Aqua give Master Xehanort the final blow since they left unfinished business in the Keyblade Graveyard.

IDK about that. Sora is the main character.

 

But your right, you never know.