I have read a few S-Tier fantasy books but never came across an S-Tier science fiction book.

I start to believe there's no such thing as an S-Tier book in that genre.

Used to build @kiwi | https://macbudkowski.com/

what fantasy do you put in the s-tier? have to disagree with you on the first dune book, that was incredible. i think ender’s game was amazing. snow crash as well was so good it ended a very popular genre at the time. but if not, cryptonomicon is as close as he gets to a completely consistently great novel.

Joe Abercrombie's First Law saga (6 out of 9 books are S-Tier imo)
Sapkowski's Witcher
Rothfuss Name of the Wind (it has boring parts but overall is a very fresh approach to the genre and has great characters)

Lord of The Rings and Hobbit sadly didn't stand the test of time - re-read them a few years ago, and they're like old Star Wars series: I like them because they're OG, but if they were published today they wouldn't become my favorites.