The PHB II is amazing. Like the poster a few up said, it's solid gold for players, and opens up some really cool new paths for their advancement.
However, my vote goes to Eberron. It's a very innovative and well-made world, and it's a welcome break from the same cookie-cutter fairy-tale world. I especially like the new takes on some of the races, such as the expansionist nature of the Valenar elves and the druidic traditions of the orcs. It makes for more interesting and more well rounded characters.