It’s mostly because of Reepicheep.

Why is a talking mouse so important? You can read my essay over at Mere Orthodoxy:

Clearly, Voyage of the Dawn Treader is the best of the Narnia novels, and a big part of that is due to a talking mouse.

Yes. I’m talking about Reepicheep, the brave, swashbuckling Narnian mouse who is on a quest to find Aslan’s country (and defend the honor of the Kings and Queens of Narnia while he’s at it.).

Sure, a lot of things happen in Voyage that also give it the claim to being the best novel, the discussion of science and modernism versus tradition and religion (although religion and science aren’t actually at loggerheads), Eustace’s Pauline conversion, the growth of Lucy, Edmund, and Caspian, and of course the quest to find the seven lost Narnian lords, which gives the entire book its shape. All of these things add up to a tightly plotted and fast moving adventure. But I think that the reason it’s the best isn’t just Reepicheep, but what he and the other characters go through in the novel, which is growing up and becoming adult Christians.

In Voyage, the things that stop our heroes and heroines are themselves, which is true in our own lives. 

Read the rest here.