Tanith Lee for so many, many reasons and so many, many books and short stories. She is the only writer I can think of who was so very, very prolific and still wrote astounding books while maintaining writing of such a high quality. Her characters are memorable, her settings vivid, and her prose beautiful without being too ornate. Perhaps my favorite author of all. She can be very dark at times, and I am not a fan of dark, but I have always been willing to let her take me there because her books are so extraordinary.
Hope Mirrlees, although she only wrote the one fantasy, Lud-in-the-Mist, but it's a classic, one of my very favorite books, and gets better every time I read it.
Sylvia Townsend Warner's short stories about the faerie kingdoms are favorites of mine. Her fairies are obsessive, sometimes kind and sometimes cruel (very often the latter), and their societies very complex. Interactions between fairies and humans (not all of her books involve these) rarely go well. I love her elegant, vivid prose. In fact, if this list tells you nothing else, you will find out that I adore elegant and vivid prose.
Patrica McKillip. Most of her plots are highly original, but sometimes she will take what you at first think is going to be a familiar story and then take it in unexpected directions. And again the gorgeous prose.