You can't go wrong with these guys:
Robert Heinlein-Sci-fi with character. Reads like a sci-fi version of a Thin Man movie if you know what that is.
H.P. Lovecraft-One of the original horror/sci-fi writers Most everything written in the genre since was 'borrowed' from Lovecraft concepts
Edgar Rice Bouroughs-Pulp sci fi/fantasy at it's manliest. Not as much depth as Heinlein but great fun to read.
Robert E. Howard-The father of the sword and sorcery genre. He created Conan the Barbarian and the Hyborian world in which he lived. If you don't feel like going out and crushing your enemy, seeing them driven before you and hearing the lamentation of their women after reading Howard....then you're just not alive.