Someone already listed Jacqueline Carey and Garth Nix, but let me plug them again.
If you are comfortable with Adult Situations in Fantasy (more graphic than most Romance novels) AND had a fascination with the Da Vinci Code (which to me, could be counted towards Fantasy/Alternate History, but that’s just me) then the Kushiel series is for you. Three books in the original trilogy, each long without being tedious, and very intriguing premise. Plus, she’s writing the second of the second trilogy right now, and isn’t too slow.
Garth Nix writes for children/teens, but his Abhorsen trilogy, beginning with Sabriel, is his finest work. I have enjoyed the “Day” series, beginning with Mister Monday, which is complex and real and set in our world with an attendant “rabbit hole world” the main character gets lost in. Excellent!
If Vampires and Adult situations are your bag, then Laurell K. Hamilton breaks genre boundaries with her Anita Blake series, beginning with Guilty Pleasures. Her writing is addictive, but “jumped the shark” with Narcissus in Chains, some say even before that. I have no problem with some of the plot choices she’s made, though many fans do, and have stopped reading due to them, but the editorial quality of her writing ALSO suffered, which to me is unacceptable.
Sharon Shinn is in the same writing group as Hamilton (that’s one way I find new authors, who do they write with? Check out author websites, like G.R.R. Martin’s for book updates and who THEY read…) and her Angels of Samaria series is complex and the world is skillfully built. If you like Pern, and the Harper Hall books in particular by McCaffrey, you might want to check out Shinn. Start with Archangel.
Let’s see, my favorite author is Sheri S. Tepper. I strongly caution you that she is a feminist writer, and has strong themes developed throughout her work. They are hard to get (out-of-print), but the Mavin Many-Shaped books are wonderful and it’s an incredible world. Start with The Song of Mavin Manyshaped, if you can find it used. The True Game, a bundling of Peter’s books, is still in print, and the three trilogies are related, all short books, but fascinating and I love her work so much it makes me ache. The Jinian series is last, but Peter or Mavin are both good entry points. And then she has many many stand-alone books but, like I said, caution: strong political themes ahead.
Patricia McKillip, but then you will know where my nickname comes from.
Robin McKinley, especially the Hero and the Crown 1st and the Blue Sword 2nd. Joan Vinge, Snow Queen/Summer Queen duology. Rhapsody: Child of Blood by Elizabeth Haydon is interesting, and there are two direct sequels, with more
I second Tamora Pierce, David Eddings (but forget the Younger Gods stuff- total crap. The Belgariad was awesome, the Mallorean was the same series re-written, but still pretty good, and the Elenium/Tamuli is only for helpless fans to get out of the library.)
There are very few authors whose books I buy in hardcover (used to not do it at all) and there are some authors who I used to snap up the day they came out and now don’t even get from the library. McCaffrey and her Acorna series is one example. Must read her early work, forget the later stuff. Piers Anthony, another good example. Xanth fell by the wayside probably long before book 16 or so when I stopped reading it.
Authors age. Their kids need braces. Misery by Stephen King is an extreme example of inside the writing process, when a psychotic fan wants more of the same…
Okay, enough expounding. I love fantasy though, and had to chime in.