After reading the first five Game of Thrones books, here is how I rate them - in order from most favorite to least favorite
1) Book 3 - A Storm of Swords - This is easily my favorite Game of Thrones book. It's also one of the best books I've ever read. In my opinion, it's the only book in the series that should be seen as being "as good as Tolkien". Martin's other books have good moments, but this book is full of them.
2) Book 1 - A Game of Thrones - This book is excellent. A great start and a captivating story. The sequences with Tyrion Lannister are especially captivating.
3) Book 5 - A Dance with Dragons - This book is good, and a definite improvement from book 4. However, I also feel it lacks the direction and "punch" of book 3.
4) Book 2 - A Clash of Kings - Aside from Tyrion's always humorous dialogue, this book wasn't great in my opinion. I couldn't bring myself to care which side won the "Clash of Kings", which was one of the main plots in the book.
5) Book 4 - A Feast for Crows - This book painted a fresh new picture of Jaime Lanister, which I enjoyed. Aside from that, I felt that it often traded plot direction for unimportant details. For that reason it is my least favorite of the series.
How do you rate the Game of Thrones books?