The Three Body Problem by Cixin Liu, both because it's reasonably solid sci-fi and because it it's Sinocentric but not propagandistic and doesn't pretend China is without flaws.

Old Man's War by John Scalzi is a solid classic

Ender's Game is a great starting Sci-Fi book. Also maybe Frankenstein? It's more horror but Shelley wrote it with the intention of it being read around a group so it would be great for book clubs.

The Metamorphosis of Prime Intellect by Roger Williams.

Do make yourself aware of the trigger warnings though, if needed for your group.

http://www.skyhunter.com/marcs/GentleSeduction.html

Kindred is the story of a mostly modern black woman who somehow keeps getting transported to rescue one of her ancestors, a white slave owner. It's written by acclaimed sci-fi writer Octavia Butler and will give you lots to talk about. It's fantastic.

Our club mostly reads short stories, and we have 180 of the stories linked here sfsfss.com/

What other scifi books do you like?

Anything by Becky Chambers. To Be Taught if Fortunate if you insist on only a single book, the two Monk and Robot books if you don’t mind it being two (they’re both fairly short at around 150 pages), and The Galaxy, and the Ground Within if you don’t mind skipping to the end of a loosely-connected quadrilogy .