DDG and ELH, but I expect to get some use from both, especially ELH when my PCs get to that level. Wheel of Time because no one in the group likes playing in "established" (aka, novel/movie) settings. Legends & Lairs: Wildscapes was the only 3rd party book that's captured my attension enough to buy, but it hasn't seen any use, yet, though it may.
If you open it up to non-d20 books, Amber Diceless tops the list. I've got a few WoD books that are in the list, too.