Well, I've read the whole Hitchhiker's series, so Adams is certainly on my list of favorites. XP
Terry Pratchett, so I'm told, is like Douglas Adams, except he writes fantasy instead of science fiction. I have yet to check out any Terry Pratchett for myself, so I'm not sure if this is true...
Also on my list is the little-known Tim Dorsey, author of a yet-unnamed series of books all about the zany adventures of historian/serial killer/proud Floridan Serge A. Storms and his (usually drugged-up) companion Coleman. (Or Lenny, depends on which books you read.) I just finished reading "The Big Bamboo" in fact, and now I'm anticipating his next book, "Hurricane Punch."
Shakespeare is quite enjoyable for me, too. The old-English style is quite appealing, and it's always fun to read out loud.

Authors!!!
Kurt Vonnegut
R.E. Howard
Karl Marx
J.R.R. Tolkein
Shakespeare
Milton
Karl Edward Wagner

and my books are...
Conan and Kull series
Tragedy of Julius Caesar
Paradise Lost
The Communist Manifesto
The Hobbit
the Kane series
Slaughterhouse Five