Frodo is the true protagonist of The Lord of the Rings. Driving sometimes up to five story lines,  Tolkien made ​​us think that Frodo had died somewhere and that forced us to accelerate our reading in order to return to his chapters. This does not downgrade Aragorn, Gandalf and the others, because you feel a connection between them, so you realize that they are buying time for Frodo to fulfill his mission. For me it is a very powerful concept that one so small could do this incredible task. Like any antihero, Frodo is not perfect, he makes mistakes and suffers from an addiction that consumes him until the end of the story. 

It is curious that Sam Gamgee (Sean Astin), unlike the other hobbits, didn't have an English accent and I don't think it was due to lack of skill, it is kind of a mistery to me. Beyond that, I think he did a good job, it was a very demanding role in the emotional and physical aspect, unlike many others parts he has played throughout his career. Definitely the most memorable.

These two are the comic relief of the trilogy, at least in the first two films. These two add value to the message of friendship that Frodo and Sam live, but the interesting thing about this relationship is that they take different paths after the chapter of Treebeard, each one growing emotionally by their own will: Merry in Rohan and Pippin in the city of Minas Tirith.

Women play a small part in the Lord of the Rings. Tolkien was writing primarily a story about war, so he had to focus on developing soldiers, allies and enemies as well as the kingdoms involved. Despite having little involvement, characters like Galadriel and Éowyn are incredibly important to the events in the story.