Here I have no uncertainty. Lord of the Rings. If you've seen the movies but not read the book and liked the movies - do yourself a favor and read the book. You can probably find the Audio book at the library. I recommend the recording by Rob Inglis.
Like I said in my Last Book Read post, I highly recommend the entire Expanse series. It's a phenomenal example of Space Opera but with solid characters and a universe that while physically small (only one solar system) feels very very large indeed. And there's a lot of scientifically driven writing - travel takes time for example!