1. E. E. "Doc" Smith - Lensman series.
This inspired Brin's uplift books, JMS' Babylon 5, and many more. Its influence in SF is comparable to that of Homer's Odyssey in general fiction. And it's a great read. If your'e in the SF book club they have a 2-volume edition of the series.
2-5 in no particular order:
David Brin ("Uplift" books, The Postman, ...)
David Drake + SM Stirling (The General series, Hammer's Slammers, Marching Through Georgia ...)
Julian May (Many Colored Land & sequels (science fantasy))
L. E. Modesitt, Jr. (Timegod, Adiamante, Hammer of Darkness ...)
Larry Niven (Ringworld, Neutron Star, other "Known Space" stories, Mote in God's Eye, ...)
Andre Norton (Time Traders, The Zero Stone, Storm over Warlock, Beast Master [inspired the cheesy movie], ...)
Orson Scott Card (Ender's Game)
Roger Zelazny (Lord of Light, My Name is Legion, Amber, ...)
And some more good ones to round out the 5
Harry Harrsion (Deathworld books, Stainless Steel Rat books, ...)
Robert Heinlein (Starship Troopers,
Frank Herbet (Dune books, Green Brain, Hellstrom's Hive, ...)
Keith Laumer (A Plague of Demons, Dinosaur Beach, Retief stories)
Fred Saberhagen (Berserker stories)
James Schmitz (Telzey & Trigger stories, Witches of Karres)
David Weber (Honor Harrington books)