Best womens one handed backhand might be Henin's. I think hers would have been very good in any era. Though my favorite one hander belonged to Catarina Lindqvist. I would have ranked hers first but her slice wasn't good enough for that. Much like early Lendl her backhand was 90% top and not enough versatility.
Another almost #1 backhand would be Mandlikova's. She often spanked winners during baseline rallies like most people hit forehands. But her slice sometimes wasn't penetrating enough which hurt her on many approaches vs. Evert. Her down the line shot was sheer beauty.
Goolagong's was gorgeous and should be mentioned. Her birds nest backhand was baffling.
Also I will mention Martina simply because her slice was so good at setting her up. Only Novotna's crosscourt slice (though less effective) can approach how penetrating Martina's slice could be.