I'm undecided between Adventurer and Warrior. Both are really great books.
On the one hand, Rogue is my favourite class, bar none. ON the other hand, D&D usually has its fair share of combat, so both books are useful. And in Adventurer, even the oriental base class was nice (Wu Jen and Shugenja are good, too, but I had those in OA already. And we don't have to lose a single word about that Samurai abomination).
Divine and Arcane had good stuff, too: Warlock and Favoured Soul as base classes (and, to some extend, Spirit Shaman and War Mage), a lot of the other material. But they can't hold a candle to the other two.