I can't remember exactly, but there was a part in the cartoon LOTR where Sam imagines what it'd be like if he had the ring. He decides that he isn't worthy to wear the ring, and actually has the willpower to not put it on. This is a powerful moment (left out of the recent movie) that really demonstrates how much integrity and strength Sam has.
Also, Sam continues to choose to help out Frodo, despite being given the chance to go home (whereas Frodo is more or less "stuck" with the responsibility to go on). So, I say Sam.
I voted for Sam but Eowyn was a very close second for me. I sat there watching it thinking I'd never have the kind of courage either of them showed, if I was called upon for it.
I'm that single vote up there for Aragorn. Eowyn was very brave herself, and Sam most definitly pulled through in the end. But I had to go for consistancy, and Aragorn definitly has this. One definitly gets the idea that no matter the situation he finds himself in he wont hesitate to do the right thing.
LOL, and you forgot to list Merry, he was right along side with Eowyn and actually stabbed the Dark Lord!
I voted for Sam for his loyalty and devotion to Frodo and the quest, but I think Merry and Eowyn are a close 2nd.
Why? He is the solid grounding for Frodo. He is the character that must discover how to support Frodo, emotionally, mentally and psychologically. This is certainly not typical of the behaviour of Hobbits.
And also in the events at the end of the Return of the King, (the book, not, the movie), Sam shows incredible leadership and determination. I won't mention what as it is sort of a spoiler for those who have only watched the movies and are still to watch the third installment.
He is the only character in the Lord of the Rings to have posessed the ring and not wear it.