A Game of Thrones by George R.R. Martin
My rating: 4 of 5 stars
Dynamite, the most compellingly complete "world" I've read in I-can't-remember. The world is brilliant, a gritty and "realistic" medieval-ish place with slowly-introduced fantastical elements -- summers and winters last for years (and even decades), there were once dragons, there are zombies (wights... same thing). Martin's tone is pitch-perfect, too, with vivid descriptions that never overstep into the sentimental.
The characters are the weak point, but that's only noticeable because one character (Tyrion the Imp) is fully realized, complex, and the flatness of the others is apparent in contrast. On the other hand, there's a certain amount of "people are the way they are because they're duty-bound and locked into roles," so maybe those characters will flower in later books.