Great idea and an interesting list, Ian. A little surprised that
The Sword of Aldones, Marion Zimmer Bradley (1962)
didn't make your list, and I might even argue a case for:
Alien Earth, Megan Lindholm (1992)
Otherwise, this is pretty comprehensive, I reckon (okay, I could be picky about one or two of the titles, preferring an alternative book by the same author, but what the heck?)
Using the bold for those I've read and italics for those I own but haven't yet system, my 'score' on your existing list is as follows:
1 *Frankenstein, Mary Shelley (1818)
7 The Sword of Rhiannon, Leigh Brackett (1953)
9 Memoirs of a Spacewoman, Naomi Mitchison (1962)
10 Witch World, Andre Norton (1963)
15 *The Dispossessed, Ursula K Le Guin (1974)
17 *The Female Man, Joana Russ (1975)
20 *Floating Worlds, Cecelia Holland (1976)
21 *Where Late the Sweet Birds Sang, Kate Wilhelm (1976)
23 Dreamsnake, Vonda N McIntyre (1978)
28 The Snow Queen, Joan D Vinge (1980)
30 The Silver Metal Lover, Tanith Lee (1981)
31 The Many-Coloured Land [Saga of the Exiles], Julian May (1981)
33 The Crystal Singer, Anne McCaffrey (1982)
35 The Handmaid’s Tale, Margaret Atwood (1985)
42 The Journal of Nicholas the American, Leigh Kennedy (1986)
47 Kairos, Gwyneth Jones (1988)
48 Cyteen , CJ Cherryh (1988)
53 Grass, Sheri S Tepper (1989)
56 The Archivist, Gill Alderman (1989)
60 Polar City Blues, Katharine Kerr (1990)
63 Beggars in Spain [Sleepless trilogy], Nancy Kress (1991)
65 Hermetech, Storm Constantine (1991)
67 Fools, Pat Cadigan (1992)
69 Lost Futures, Lisa Tuttle (1992)
70 Doomsday Book, Connie Willis (1992)
77 Primary Inversion [Skolian Saga], Catherine Asaro (1995)
79 The Sparrow, Mary Doria Russell (1996)
80 Memory [Vorkosigan series], Lois McMaster Bujold (1996)
81 Remnant Population, Elizabeth Moon (1996)
87 Vast, Linda Nagata (1998)
89 Brown Girl in the Ring, Nalo Hopkinson (1998)
90 Dreaming in Smoke, Tricia Sullivan (1999)
91 Ash: A Secret History, Mary Gentle (2000)