Yes, indeed. Dune. Lets talk about the why. Its much more than an average fantasy because it contains a lot of angles. It is full with psychology, politics, and sociology. Specifically, it touches on subjects like discrimination & racism, oppression/tyranny, managing, infiltrating, warfare, and various eastern philosophies (since the Atreides are forced to suffer). Did I forget anything? Please, do improve my answer! :)
I'm allergic to clumsy exposition and Herbert, by and large, did a very good job.
Many times you can read what the characters are thinking. This gives you many situations where you get a sense just how tormented someone feels, but how he needs to react in order not to lose control.
Fremen suffering in general. How it shapes their rituals, culture and further development of the story. Interesting also how it clashes with other cultures. There's also a nice part about how they must continue to suffer along, lest they become weak.
I mostly read non-fiction myself, so, just like you, I looked for something else amd randomly chose Dune. Loved it.
Brilliant world building and a story arch that keeps on giving. And there are 7(?) books in total.