Cordwainer Smith;
H Beam Piper;
Marion Zimmer Bradley.
The ones that I do know divide into four groups:
British
Wells
Stapledon
Aldiss
RC Churchill (not well known)
Campbell-edited
Heinlein
Asimov
Blish
Anderson
An American sequence (overlapping with "Campbell")
Heinlein
Anderson (2)
Niven
Pournelle
Anderson's later future histories
six or seven
The magnitude of Poul Anderson's contributions is evident from these lists.
