Best fantasy of all time IMNSHO is Ursula LeGuin's Earthsea books. Technically they are for teenagers, but they are vastly better written than just about all of the books listed here so far. They are also proof that a fantasy's worth CANNOT be judged by its weight.
Best series still in production is George R R Martin's Song of Ice and Fire mentioned above. Four books out so far, one in production, two more expected, plus a book of short stories set in the same land also on its way. Stunningly good and so far ahead of most other fantasy it's not funny.
Barbara Hambly is always worth reading, but don't expect traditional fantasy - she is very much the revisionist wing.
Elizabeth Moon's Paksinarrion trilogy are great as well, but again, revisionist. Thanks to whoever it was here who recommended her books.
Guy Gavriel Kay is probably the consistantly best writer. If you prefer traditional fantasy then you need the Fionavar Tapestry trilogy; later works are fantasy re-writes of real historical events - which are better than the idea might sound.
David Eddings early books are good (unlike most fantasy writers he has a sense of humour), but later books just copy the early ones.
David Gemmell wrote one good book (Legend) then wrote the same book over and over again.
Simon R Green's fantasies are a hoot, but don't expect literature.
China Mieville writes very good, unusual books.
M
i think one of the dons is abusing his power! i demand justice that is swift yet brutal 
