Edgar Allen Poe
Mary Shelley
Bram Stoker
Robert McCammon
Stephen King
Peter Straub
H.P. Lovecraft
Joe Hill
Clive Barker

Not in any order....

Stephen King
Jack Ketchum
H.P. Lovecraft
Ray Bradbury
Peter Straub
Clive Barker
Ramsey Campbell

I notice a few people listed Ray Bradbury. I've always considered his work to be more science fiction than horror.

In my opinion, a good deal of Bradbury's work could be classified as horror, or even "light" horror. He isn't outright scary or gruesome, but a lot of his stories have an underlying, unsettling creepiness to them that I think would place them in the horror genre. Even if it's just barely. :)

1 - Blackwood
2 - Matheson
3 - Lovecraft
4 - Rice
5 - Koontz
6 - King
7 - Herbert
8 - Stoker
9 - Poe
10 - Laymon

As Ellie said, a lot of his work has horror elements and a good deal of his early work is straight up horror. His work appeared in Weird Tales and other horror pulps of the era. Something Wicked This Way Comes is definitely a horror story, and the collection October Country has many terrifying tales in it.

I'll make my ranked list in a few days.

Mary Shelley
Bram Stoker
H.P. Lovecraft
Edgar Allan Poe
M R James
Richard Matheson
Ray Bradbury
Robert McCammon
Peter Straub
Jack Ketchum

1.HP Lovecraft
2.Clive Barker
3.Edgar Allan Poe
4.Ramsey Campbell
5.James Herbert
6.Stephen King
7.MR James
8.Peter Straub
9.Anne Rice
10.Dean Koontz

1. Lovecraft
2. Poe
3. Blackwood
4. Bradbury
5. Stephen King
6. Mary Shelley
7. Bram Stoker
8. Sheridan La Fanu
9. Frank Belknap Long
10. William Burroughs

And the rest worthy of 1/2 point, as per V's instructions:

Clive Barker
MR James
Richard Matheson
Shirley Jackson
Robert Bloch
Jack Ketchum
Clark Ashton Smith
HH Munro
Peter Straub
Whitley Streiber (whether you consider him a lunatic or a clever publicist of his work post Communion, it's good to remember he wrote some really great horror before that, such as The Hunger and Wolfen)
Franz Kafka

I was told by Neverending I should come by and offer up my ten, focusing on ones you guys might miss.

1. Dante Alighieri
2.Franz Kafka
3. Arthur Machen
4. Brett Easton Ellis
5. William Burroughs
6. Joyce Carol Oates
7.Joe Lansdale
8. Thomas Ligotti
9.Robert W. Chambers
10. William Hope Hodgson

1/2 point

John Skipp
M.R James
Goethe
The Marquis De Sade
William Peter Blatty
Ira Levin
Harlan Ellison