

At least in 2013 when I started using Steam more seriously if your connection dropped it would prompt you asking if you wanted to switch to offline mode. And I know this because I had Steam on a laptop that I carried in my bag hibernating and I didn’t had internet in some places I went to. So that has been fixed for over a decade.
First of all my answer would be that I don’t know. If I had to choose one I would probably pick the end of eternity, but mostly because it was an excellent book if you’ve read the Empire and Foundation books before, so maybe I should pick the Foundation trilogy… But if you ask me for my favorite sci-fi story I would almost assuredly pick SOMA.
Which takes me to that you should probably ask what’s your favorite sci-fi story instead of book. Some people prefer games or movies, and even people like me who enjoy reading might just not have read enough sci-fi to pick a favorite book (I for example have mostly only read Asimov).