Edgar Pangborn
C. L. Moore & Henry Kuttner
Fritz Leiber (for sf or fantasy, or horror/urban fantasy for that matter)
Fred Brown
Leigh Brackett
Theodore Sturgeon
Avram Davidson
Damon Knight
Roger Zelazny
Eric Frank Russell
Cordwainer Smith
Clark Ashton Smith
Great list. I agree that short fiction is the way to go, especially when sampling but, if I had to pick some novels for that list (because some people just go for novels), I'd say
Judgment Night for Moore, either
Fury (the traditional pick) or
The Valley of the Flame for Kuttner, and
Earth's Last Citadel for Kuttner and Moore.
For Leiber's SF, it's tough, but I might give
The Big Time the edge. For fantasy, also tough, but I might say
Our Lady of Darkness as long as I could say
You're All Alone/
The Sinful Ones for Just Plain Weird stuff. Actually, the only Leiber I wouldn't recommend would be
A Specter Is Haunting Texas and, especially,
The Silver Eggheads. Brown's
What Mad Universe is great. For Brackett, it's got to be
the Stark books but
The Long Tomorrow is really good if you're looking for more sober SF. Sturgeon: it's hard not to say
More Than Human. but you can't go wrong with any Sturgeon. Zelazny:
This Immortal. Russell:
The Great Explosion, unless you count the collection of connected stories that is
Men, Martians, and Machines. (Fans of Russell's more highly regarded novels will yell at me, but those didn't work for me so well.) Cordwainer Smith has just the one with
Norstrilia. I don't think CAS has a novel. I have Pangborn's
A Mirror for Observers, in the Pile, rather than
Davy, so I hope that's also a good one. I haven't read any books by Knight that do it for me (or any books of Davidson at all), so I can't say there.
For additional suggestions I'd say almost all of Asimov and Clarke, of course. Aside from that, I don't think these books up to 1980 have been mentioned yet:
Poul Anderson - Tau Zero
Ben Bova - The Kinsman Saga
Algis Budrys - Rogue Moon
C. J. Cherryh - The Faded Sun trilogy
Hal Clement - Mission of Gravity
L. Sprague de Camp - Lest Darkness Fall
Edmond Hamilton - The Star Kings
Pohl & Kornbluth - The Space Merchants
Frederik Pohl - Gateway
James H. Schmitz - The Demon Breed
Clifford D. Simak - Way Station
A. E. van Vogt - Mission to the Stars/The Mixed-Men
I'm cutting off at 1980, so I'm also including 1980 and making it a baker's dozen
- Robert L. Forward's
Dragon's Egg.
If you're into short fiction, then all these authors have written great stories. Additionally, there's obviously all of the Ballantine "The Best of" series and Asimov/Greenberg's "The Great SF Stories" series. And I couldn't live without essentially all of Varley's and Tiptree's stories.