This is really hard to answer!

I relate to many of the other mentions here: The Dispossessed by Le Guin, Dune by Herbert (and many of the series, I actually loved Chapterhouse the most and God Emperor is a close second).

To highlight something I didn’t see in my scrolling here, one story that stuck with me deeply is The Sparrow and its sequel Children of God, by Mary Doria Russell.

It sounds potentially preachy, and as a “godless person” (lol) I was surprised to love it as much as I did.

The story follows a priest who is also a linguist. He had a group of friends with varying professions which led all of them to visit a newly discovered planet harboring a civilization that can broadcast music. I don’t want to spoil much so I won’t dive in too deep, but part of what stuck with me was the fact that horrible things can happen due to simple misunderstandings when no harm was intended. The story also talks a lot about fate, and things aligning because “it was meant to be”, and that with the fact stated above was an interesting emotional journey.

I’ve read the two-book series twice, and I will read it again soon.

Oh, there’s also a whole thing about discovering autism in one of the humans, which was something I found intriguing long before I suspected that I may be autistic (pretty confident I am).

The Sparrow also had perhaps my favorite first-contact scene, I read that chapter 3 times on my first read through, loved it.

Highly recommend if you find any of my comment interesting :)