For me one of the best SF writer is Karel Chapek with his "Factory of absolute".(I'm sorry for my translation, I don't know how this book called in English)
Have you read it? What do you think about it?
Strugazkys are genius writers,"Monday starts on Saturday" is my "table-book", but they aren't science-fictioneers, they wrote social fiction. If our topic is "Lem an other SF", it's uncorrectly to compare Lem with Strugazkys.
Bradbury is a fantasy.
Garrison, Shekley, Simak ,Clark - it's fairy tales wits "science" decoration.
It's seemed to me, that Lem is only writer (exept Stapedon, Chapek and Wells), whom we can called " the author of science fiction".
I don't think, that other books are bad, but it's another genre, not SF.