The third and, in my opinion, the strongest of the Fionavar series, for the simple reason that GGK can write a climax of multiple plot threads like nobody's business.
This isn't to say that this book is flawless, of course. As in the rest of the series, GGK occasionally waxes pretentious on his characters' behalf (which they can do just fine without his help), forgets that the seventh time a supposedly stoic character weeps in as many days it takes away some of the emotional power of the scene (weeping is fine; asking us to find it just as overwhelmingly heartbreaking every time isn't), and is way too emotionally invested in epic love triangles, and in having everyone find them the SADDEST THING EVER, including characters who have experienced much more awful things in their lives. To counterbalance that, though, he writes some genuinely beautiful moments and some genuinely original twists on the standard tropes of epic Eurofantasy. This series has one of my favorite twists on the "Elves sailing to the West" trope (redeeming, for me, the otherwise pointless lios alfar), and some fun uses of the aforementioned epic love triangle (Arthurian, no less). GGK writes engaging characters, when he's not masking that with pompousness about their pain, and for all my mostly affectionate mocking of this series, the last half of this book is when everything starts coming together in a tumbling rush of resolving plot, and I remember how worth it the payoff is. This is no Lord of the Rings, and very deliberately not, but it's a lot of fun as a quick-reading fantasy trilogy.
(Warnings for triggers about rape and rape survival, but that's mostly in the first and second books, so it seems a bit silly to mention it here. I don't plan to go back and review all three, though, so I'll do it anyway.)