Book 3 (Storm of Swords) probably ranks with Book 1 (Game of Thrones) in a tie for my favorites of the series (note: I have only read up to book three thus far). The plot stays fairly tight throughout, the twists and turns are relatively fair to the reader without being too obvious and Martin is refreshingly comfortable with killing off a major character or two when it works for the story.
Also I enjoyed the tapestry of this world that Martin is creating. We are learning more and more about legends of the Night’s Watch, the bizarre customs of the eastern realms, and the evil of the Others.
In fact, although I’ve said previously that Martin has written too many story lines into his saga, I must commend him for keeping each one moving forward. This is something that few writers seem to be able to do. For example: Robert Jordan, author of the Wheel of Time series, became seriously bogged down by his multiplying plot lines. So much so that after over 10 books his once promising fantasy series is still unfinished and not projected to be until three more books are released. With Jordan, it got to the point that each character would take up enough of each book that nothing would happen for 900 pages.
Fortunately, Martin keeps his tale in control.