Here's a taste:
"Ill met in Lankhmar" tops any list. How could it not? Fafhrd and the Gray Mouser defined sword & sorcery for me as a child, and I'm thrilled that, having just started rereading their adventures they are thus far holding up. Michael Moorcock's "Stormbringer" is tied or a close second. I haven't read that since I was 15 but the Moorcock I have read hasn't dated. Basically, you don't know s&s without Leiber and Moorcock.
Howard's "The Frost Giant's Daughter" reads like ancient myth, and is my favorite of the Conan tales. Finally CL Moore's "Black God's Kiss", which I only discovered as an adult, mesmerized me with its imagery, an amazing hybrid of Howard's action with Lovecraft's imagery that reminded me that s&s got its start in Weird Tales and made me want more Old Weird in contemporary S&S (and more s&s in contemporary Weird Tales!).
And speaking of contemporary, I love James Enge and Scott Lynch for the way they evoke emotions in me now the way Leiber did when I was just beginning to explore the subgenre. Of course, I edit one of them, but I highly recommend both. I also edit Mark Chadbourn's Swords of Albion in the US, chronicling the adventures of an Elizabethan James Bond in a Cold War struggle with the Fae. It's quintessential S&S that should take its place in the canon in time.
Nor can I let the opportunity to shamelessly plug Swords & Dark Magic go by. Co-edited with Jonathan Strahan, it's our forthcoming S&S anthology of all original tales from writers like Steven Erikson, Scott Lynch, Joe Abercrombie, Gene Wolf, Glen Cook, Michael Moorcock, CJ Cherryh, Tanith Lee, Robert Silverberg, Greg Keyes...
Okay, I'll stop but obviously S&S has been on the brain here lately. Glad it's making a resurgence.
There are also lots of great lists in the comments.
Personally, my favourite in the genre is Karl Edward Wagner's Kane books -- dark, violent and compelling, they make for a great read. They are very much sword and sorcery for adults tastes and preoccupations.