These are not all classic fantasy, as some spill into sci-fi and other genres, but they are great on the imagination/world building/classics from my childhood and today.
Patricia C. Wrede
Anne McCaffrey : specifically her Tower and hive series, Moretta, and Dragonflight (the first book in the Pern world)
David Weber
Eric Flint 1632 - GREAT book.
LKH : but only the Anita Blake series through book 4 or 5. After that they majorly suck and could be used as a guide on how NOT to write. First couple were brilliant though.
J.D. Robb : These are technically romance, but they are set in the future. I only point them out because I think they are a really different and great writing style from some of the heavy thick tomes I listed before, so they are a nice counterpoint.
Piers Anthony : Funny, quick, but I can only take so many punny, punny, puns.
His first four Incantations of Immortality series are FANTASTIC. Don't know about the rest, though I just saw a new one came out. (anyone read it? Is it anygood? let me know!)
Robert Aspirin Myth books are cute
R A Salvatore His Drizzt series starts with Homeland. I made it about 10 books in, got tired of it.
Tad Williams : Great books but very heavy reading
Robert Jordan : Wheel of Time series through book 7-8 is good. I read them consecutively and got overloaded, lol.
Brian Jacques : Redwall is a true classic
Not an author (I will get to them,
) but one of my favorite all time books.
Good Omens.
Neil Gaiman - Awesomely good
Terry Pratchett - Also awesomely good, and funny
Hmmmm....I am sure I have a million more. You should probably ask in the sci/fi fantasy forum. They might even have a running list somewhere they could direct you to.
ETA: Duh!
Simon R. Green. I am reading his Nightstalker series right now, very quick, fun, dark, nitty gritty books. Almost Film Noir but for books. Book Noir? Anyways. Good books!