Out of the first three, I liked `the revenge' the most. The story took some interesting swings, and the many intertwined storylines were not as easily untangled as in the earlier books. Also, it's the first one with pretty much an open ending, leaving the reader at the edge of his seat waiting for the next one to get published (the fate of the Dwarves).
I'm reading the English translations of the books, and I was a bit put off by the change in translation of the älf-race. In the first books, the singular form was ``älf'' and the plural ``älfar'', which I liked and got used to. In this edition a single älf was also being referred to as an älfar.