UA tops the list.  There are some mighty fine ideas in that book.
XPH comes next.  While I don't like the "new age" feel of 3E psionics, that's easy to work around, and the system is bloody solid.
ECS is next because it's a well laid-out setting that I like.  Normally, I home brew but it's good to have a low-work option around.  Plus, there are some excellent ideas in the book.
ToM has some really, really cool ideas in it that fit nicely into a couple of things I wanted to do with my homebrew, anyway.  At least personally, that puts it in the category of things I'd keep, even if the rest went on auction to pay medical bills (or some such).
While I enjoy many of the other books, they don't qualify as necessary by any stretch.