Stanislaw Lem (1921-2006)
Though I've only read him in translation, Stanislaw Lem was one of my very favorite writers in any genre or language. His astonishing body of work was mostly a response to the horrors and absurdities of the 20th century, and I am sorry to say that it is still relevant to our age. Though he was a cranky pessimist to the end, he expressed it in his fiction with an exuberance of imagination unmatched since Olaf Stapledon, and often with a wicked, hyperintellectual sense of humor. When I stumbled on The Cyberiad in my youth, I was never the same again.
If you haven't been introduced to Lem's work, my own Lemography is one place to start, as is Lem's official web site.