I vote for Sam; because while he and Bilbo are the only two beings to voluntarily give up the One Ring, he went further, taking the Ring
while knowing the risks, in order to stop it falling into the hands of Sauron. It is no wonder that the relevant chapter is called
The Choices of Master Samwise.
He then resisted the call of the Ring, which was offering to make him 'Samwise the Strong', due to his 'hobbit common sense'. Sam then, after rescuing Frodo, voluntarily gave the Ring back to him. In doing all this, he ensured that the War of the Ring was won.