best? i've read so many good books i don't know that i have a best book.
here are some of my favorites though
Dune, and not just the 1st book, but the whole series, even the ones by Frank Herbert's son and that other guy (can't remember their names now).
Fafhrd and the Gray Mouser series by Fritz Lieber
The Coming of Conan the Cimmerian, The Bloody Crown of Conan, and The Conquering Sword of Conan (compilation of the original Conan stories by Robert E. Howard)
The Savage Tales of Solomon Kane (compilation of the Solomon Kane stories by Robert E. Howard) - I haven't read anything by Robert E. Howard that I haven't liked yet so just read anything by him.
Anything by Terry Pratchett, especially the Discworld books.
Good Omens by Terry Pratchett and Neil Gaiman
The Lensman and the Skylark series by E. E. "Doc" Smith
Anything by William Gibson - Neuromancer is probably his best known book
Pretty much anything by Neal Stephenson. Haven't read his newer series yet though.
Tad Williams is another great author.