I’m all over the place, but among my favorites:
Larry Niven / Jerry Pournelle
Heinlein
Asimov
Jack Chalker
Phillip Jose Farmer
David Drake
Dan Simmons (The Hyperion books are superb!)
Stephen R Donaldson
Tolkien (goes without saying)
Orson Scott Card
Dennis L Mckiernan
Stephan Baxter
For easier, lighter reading like when I’m flying, I tend towards:
Terry Brooks
Piers Anthony
Gardner Dozois and his various short compilations
For unreadable Dreck, any book by:
Kim Stanley Robinson
I wish Dozois was more prolific.