Top of my list, and possibly the most underrated/underread book I've ever read. It's incredible but so few people know of it. (The fact that it's been mentioned here fills me with joy). Beautifully written, wonderful story, intriguing (soft) magic system, great characters, some unusual twists.
So my list would probably look something like
1) the Riddle Master trilogy, as mentioned above
2) His Dark Materials, by Philip Pullman.
it gets surprisingly difficult to fill the last 3 slots, because there are so many I've read and loved for such different reasons...
3) I'm tempted to put Stormlight here, but as it's unfinished I'm not sure. Mistborn can sit in as a placeholder.
4) the Daughter of Smoke and Bone series by Laini Taylor. A different fantasy sub-genre, but very good. I particularly enjoy the price of magic in these, because nothing is more fun than watching characters suffer
5) The Young Wizards series (Diane Duane), if only because it's such a wonderfully different take on magic, and one that greatly appeals to a science geek like me. Loved these when I was younger, and was pleasantly surprised on recently re-reading.