Here is a short list of my favorite authors/series.  Dune and the Wheel of Time would be on this list, but we've already talked about those.

Author: J. R. R. Tolkien
Series: The Lord of the Rings
Books:
        The Silmarillion
        The Lost Tales (volumes 1-12)

Without question, The Silmarillion is the greatest single work of fiction that I have ever encountered.  The detail of the world is unbelievable -- especially when you follow it up with 'Unfinished Tales' and the series of 'Lost Tales' books.  The Lord of the Rings trilogy is also great, but it doesn't even come close to his masterpiece IMHO.

Author: George R. R. Martin
Series: A Song of Ice and Fire
Books:
   A Game of Thrones
   A Clash of Kings
   A Storm of Swords

This is my current favorite series.  There are only 3 books in the series now, but the world and plot are larger and more detailed than even The Wheel of Time and I wouldn't be surprised to see this grow to 8 or more books.  Moreover, where Robert Jordan is a great pulp author (remember his old Conan books?), Martin is a true literary talent like Tolkien.

Author: Margaret Weiss and Tracy Hickman
Series: Dragonlance Chronicles (1st trilogy), Dragonlance Legends (2nd trilogy)
Books:
   Dragons of Autumn Twilight
   Dragons of Winter Night
   Dragons of Spring Dawning

   Time of the Twins
   War of the Twins
   Test of the Twins

   Second Generation 
   Dragons of Summer Flame

These two trilogies are classic D&D fantasy.  Don't look for a literary masterpiece here, but the characters, world, and endings are some of the best I've ever seen.  I've patterned my life after Raistlin ever since elementary school ;)  Don't be distracted by the hordes of spin-off Dragonlance books written by other authors.  Start with 'Dragons of Autumn Twilight' by Weiss/Hickman and you'll be happy.  There is a book of short stories titled 'The Second Generation' that is a great follow-up to the trilogies.  However, the recently released 4th book in the 1st trilogy, Dragons of Summer Flame, was an unbelievable disappointment.  Remember how I mentioned that the endings of the trilogies were some of the best I'd seen?  Well, tacking on the fourth book completely destroyed that...

Author: Tad Williams
Series: Memory, Sorrow, and Thorn
Books:
   The Dragonbone Chair
   The Stone of Farewell
   To Green Angel Tower (vol. 1)
   To Green Angel Tower (vol. 2)

I love this series.  It starts fairly slow (I actually started counting the number of fantasy cliches on each page), but by the time I finished the first book, I was in love.

Author: Katharine Kerr
Series: Deverry
Books:
   Daggerspell
   Darkspell
   The Bristling Wood
   The Dragon Revenant

   A Time of Exile
   A Time of Omens
   Days of Air and Darkness
   Days of Blood and Fire

   The Red Wyvern
   The Black Raven
   The Fire Dragon

This is organized into 3 sets of 4 books.  The plots tend to repeat in the more recent books, but that just shows you that they were good enough to duplicate ;)  Start with Daggerspell and see if the genre is to your taste.  This is pure Celtic fantasy -- a personal favorite of mine.

Author: C.S. Friedman
Series: Coldfire Trilogy
Books:
   Black Sun Rising
   When True Night Falls
   Crown of Shadows

This is a VERY dark series with great characters.  I don't understand why nobody has ever heard of this series...

Author: Raymond Feist
Series: Krondor
Books:
   Magician: Apprentice
   Magician: Master
   Silverthorn
   A Darkness at Sethanon

   many others

I really liked the first set of 4 books.  Feist has added many more to the list, but I'd start with the best and work my way down if I were you.  This is more classic fantasy (notice a pattern yet?).

-- MattWalsh - 25 Jan 2002