I guess the quick answer would be Frodo for carrying the ring and the burden associated with it. My pick would be Sam.
The guy was the best sidekick in movie history. He accompanied Frodo the whole way, even carried the ring when he thought Frodo had been killed (after being kicked out for being right), saved Frodo from being held captive, and was there at the end when Frodo wigged out.
Thoughts?
Sure...Frodo without Sam replaced by another hobbit MIGHT have been able to get it done. Unlikely, but if Frodo was replaced with another hobbit, perhaps the job is done.
But can you replace Gandolf? I just don't think so...he seems special to me! Going along with this thinking, I vote Gandolf.
That said --- if I was a writer who got to put my MVP list before entertaining a different character just because Marillac got me thinking it would have gone like this:
1. Frodo
2. Gandolf
3. Aragon
4. Samwise
5. Gollum --- lead them when they had no idea where to go, even if he tried to sabotage! Also helped Sam/Frodo realize how powerful the ring was!