I don't think that there's one "true" hero. It's more of a culmination of several people's herioc acts. So many people had a heroic part in the LotR. For instance:
- Boromir was heroic in his defence of the Hobbits when the orcs attacked at the end of the Fellowship.
- Gandalf sacrificed himself to the balrog in Moria.
- Aragorn travels the Path of the Dead and saves the day.
- Eowyn and Merry march off to battle and slay the witch-king.
- Pippin goes to battle and saves someone's (his name I can't remember at the moment) life.
- Frodo bears the ring to the fiery casms of Mt Doom.
- Sam saves Frodo on several occasions.
- Legolas and Gimli try track Merry and Pippin in an attempt to save them.
- Theoden fights bravely to help Gondor.
- Faramir resists the temptation of the ring and allows Frodo to continue his task.
- So many others that I can't think of right now.
Hence, no one person was the "true" hero per say, it was many people that worked together to complete the monumental task.