With a few exceptions (Q, U and X) I’ve limited it to authors I’ve read. And HA! I’m the first one to post the whole alphabet! 
A is for Aspirin, Robert who greatness was once Myth-ical.
B is for Brown, Fredrick. One of the two greatest funny SF short story authors (see “S” for the other)
C is for Campbell, John W. Decent SF author, best SF editor. Ever.
D is for De Camp, L. Sprague who gave us the “The Wheels of If” and Lest Darkness Fall
E is for Evans, E. Everett who wrote the bizarre Man of Many Minds, an EXPENSIVE sequel I’ve never seen and not much else.
F is for Finney, Jack The Body Snatchers and “The Third Level”
G is for Gernsback, Hugo who wrote Ralph 124C41+. Ever hear about the Hugo Awards? They were named for him (mainly for his editing skills). He also makes British SF reviewer psychotic because he “stole” science fiction from the Brits.
H is for Harrison, Harry who wrote Deathworld and Bill the Galactic Hero
I is for Ing, Dean who wrote so-so futuristic technothrillers long before Tom Clancy ever hunted for Red October.
J is for Jones, Diana Wynne, who’s stuff will appeal to any Harry Potter fan (try Archer’s Goon )
K is for Kuttner, Henry with his wife ( “M”, below) wrote a ton of great SF stories (“The Proud Robot” to name one)
L is for Leinster, Murray who predicted the internet in 1948. (“A Logic Called Joe”) and a bunch other stories. (or Laumer, Keith. I’m torn)
M is for Moore, C.L. Best (IMO) of the first generation of female SF authors. “Fruit of Knowledge” is one of her best short stories.
N is for Nourse, Alan E. Blade Runner (no relation) and other SF stories, many of which have a medical theme
O is for Oliver, Chad. I’ve got four books by him and haven’t gotten to any of them.
P is for Pangborn, Edgar “I’m Davy, who was king for a time. King of the Fools, and that takes wisdom”
Q is for Quick, T. W. Again, I’ve got two books by him, and I’ve never read 'em
R is for Russell, Eric Frank. Wasp and Next of Kin and lots of others.
S is for Sheckley, Robert. One of the two consistantly funny S.F. short story authors ever. (Fredric Brown is the other)
T is for Tenn, William. Another great SF short story writer.
U is for Uttley, Alison. See “Q”
V is for Vinge, Vernor. Best space-opera writer still writing SF. Won two recent Hugos (A Fire Across the Deep
W is for Watt-Evans, Lawerence. Great underrated SF/F author. Denner’s Wreck is a wonderful reexamination of Lord of Light and his Esthar series is tons of fun ( The Misenchanted Sword is a good starter.
X is for Xue, Can According to the ISFDB, one of only 3 SF writers who’s last name begins with “X”. I know nothing about him/her but she/he wrote the most
Y is for Yolen, Jane. Vaguely LeGuin-ish stort stories. Vaguely
Z is for Zebrowski, George Killing Star. Why broadcasting militarist SF is a bad idea.