What about:
* Andre Norton?
* Poul Anderson?
* H. Beam Piper?
* A.E. Van Vogt?
* Norman Spinrad?
* Andre Norton?
>>Great writer—couldn't get enough of her when I was a teen. She's close to the top ten.
* Poul Anderson?
>>I put him in my top ten.
* H. Beam Piper?
>>Never heard of him.
* A.E. Van Vogt?
>>I like him. He wrote a great time travel book who's title escapes me, and the “Weapon Shops of Ishar”.
* Norman Spinrad?”
>>Never heard of him
Since you mentioned other authors, don't forget,
John Wyndham - The “Chrysalids” and “The Day of the Triffids” are great. He is near the top ten too.
Ursula K. LeGuin - both fantasy and SF author, she is also of top ten caliber.
I find it hard to distinguish among them after the top five or so. My criteria are:
1. Enjoyable - Do I enjoy everything they write? Most? Can I read the same book over and over? This latter quality is quite rare, since I always remember the plot, even after twenty or thirty years.
2. Historic - Are their works notable 30, 50, 100 years later?
3. Popular - Do millions of people like their works?
4. Prescient - Do their writings anticipate scientific or social developments?