The Fellowship of the Ring
The hero of the novel gets complicated in The Two Towers and The Return of the King, when the series splits into separate narrative arcs for each front of the war against Sauron. But as of The Fellowship of the Ring, the protagonist is definitely Frodo Baggins. He is the one who inherits the Ring of Power and accepts the unpleasant duty of carrying the thing from the Shire all the way to Sauron's stronghold in Mordor. All of other characters in the book enter into the narrative only to assist Frodo, which indicates that this little Hobbit is our central character. Tune in to The Two Towers to see how this starts to change.