For science fiction, Dune is a must, and Children of Time is probably the most mind-bending sci-fi I've read since. Both create imaginary worlds that are so detailed and plausible that they make you see our world in a new light.
Interesting to see what they'll do in the upcoming movie / series, whichever it was though. It's great in terms of worldbuilding, and it's influenced a lot of other stuff (like Warhammer 50K).
It's difficult not to see modern politics in that light afterwards, with its cult around personalities rather than ideas.
Reading past that is, truly, not worth it. Story's over, stop reading.
And then there is Whipping Star, which is best skimmed through so that you can get to The Dosadi Experiment.
In fairness, Whipping Star is a fabulous take on what it would be like to attempt to communicate with an alien species that does not share our basic assumptions.
Dosadi Experiment is filled with fascinating ideas. Little gems like: “Does a population have informed consent when that population is not taught the inner workings of its monetary system, and then is drawn, all unknowing, into economic adventures?”