I allways promote Stanisław Lem, mainly Solaris.
Quick explanation: small group of scientists trying to study a possibly-intelligent ocean (called Solaris).
Why I like it: the worldbuilding is done by providing basic facts about Solaris, plus a lot of Solaristic theories, which creates space for your imagination and new theories. The interactions between humans are realistic and have meaningful dynamics. Every character has their unique perspective and I find it worthy of re-reading from that perspective. And it’s also quite short.
I’ve also read some other Lem’s books, but while they aren’t bad, I don’t find them as excellent as Solaris, maybe with exception of Fables for Robots, The Cyberiad and Memairs found in a Bathtub.
In terms of sci-fi, I’ve got The War of the Worlds on list.