Since we're sharing favorite authors, I guess I'll give out my few.
Eco, I've already mentioned (love his novels, but the plays not so much, but he writes top-notch papers about linguistics), Tolkein as mentioned by others, and the last one relevant to the fantasy genre is George R. R. Martin- If you haven't read the Song of Ice and Fire series, stop what you're doing and go find them.
The other two of my top five are not fantasy or sci-fi, but the runner-up 6th spot goes to Dan Simmons. He wrote some amazing books in the Hyperion series (Hyperion, Hyperion Rising, Endymion, Endymion Falling) and I am reading one of he other books, Illium (as in the Illiad). He wrote some crime/mystery novels which I have not read, but they've received very favorable reviews. Runner-ups for 7th and 8th place are outside of the genres we're discussing, but at a tie for 9th place are J.K. Rowling and Phillip Pullman (Harry Potter series and His Dark Materials series, respectively).
On a related note, if you're the least bit interested in conspiracies or counter-culture, the "Illuminatus Trilogy" by Robert Anton Wilson and Robert Shea is a must read. Those books (which I found bound together in one volume) had me cracking up the whole read through.
Okay, now I've got to go check out some new icons.