I think they were all heroes. Frodo actually destroyed it, but he couldn't have done it without Gandalf, Aragorn, Sam, Merry, Pippin, Boromir, Legolas, Gimli, Elrond, Bilbo, Galadriel, Haldir, Theoden, Eomer, Arwen, Eowyn, hell, even Gollum! I think Gandalf was the most important though, he planned everything.
I think Tolkein, as an intellectual, was quite consciously exploring all the classic hero types, and the ways they can fall from grace:
The wise guide - Gandalf, Galadhriel, Saruman
The warrior king - Aragorn, Dain, Denethor, Theoden
The dashing young warrior - Eomer, Boromir (one of the biggest changes in the movie is making Aragorn young and dashing. I really like the character but it's nothing like the old competent Aragorn in the books)
The cultured warrior - Aragorn, Faramir
The champion of his people - Legolas, Gimli
The woman who transcends her female role - Eowyn, Galadhriel
The woman who is a beautiful inspiration - Arwen (yawn)
The ordinary guy who rises to the challenge - hobbits
They are absolutely all heroes. I think that Tolkein would say that to try to pick just one is missing the point.