Hard?
Nearly impossible . . . but giving you my favorites, not "best" or those with most literary merit or something else, makes it easier. So here goes:
1.
The Left Hand of Darkness – Ursula K. Le Guin
2.
Dune – Frank Herbert
3.
Restoree – Anne McCaffrey
4.
The Snow Queen – Joan D. Vinge
5.
Aegypt – John Crowley
6.
Lincoln’s Dreams – Connie Willis
7.
The Mists of Avalon – Marion Zimmer Bradley
8.
Courtship Rite – Donald Kingsbury
9.
Red Mars – Kim Stanley Robinson
Only space for one more . . . should it be
The Once and Future King, or
Rite of Passage??? Gack, um, White makes me laugh, Panshin makes me think. Laugh, think, laugh, think . . .
Dang it, go for romance instead:
10.
Partners in Necessity (three books bound as one: cheating?) – Sharon Lee and Steve Miller