Dang - only one mention of Bluffside. The subliminal messages aren't working!
I'm like Nifft - I'm not sure I even own 10 D20 books. Here's my list.
PHB/DMG/MM (don't really count)
I should caveat this with the fact that I'm not DM'ing right now, but if I were, this is the order in which the books would be most useful to me -
1. (TG/MEG)Bluffside: City on the Edge - takes ALL the work out of city-based campaigning for me. Let's face it, city-based gaming is HARD - I know. It took about 5-6 writers a year to write this book.
2. (MEG)Urban Blight - Perfect! City-based "side-trek" style encounters that I can plop into my Bluffside campaign. It doesn't get any easier than that.
3. Tie - (MEG)Blight Magic & (Mongoose)Chaos Magic - I've always wanted to tie in an alternative style of magic into an over-arching campaign. I like both of these equally well.
4. (Bastion)Alchemy & Herbalists - The book itself is not laid out real well, and some of the stuff is kind of weird, but I really like some of it, and I could see it adding a lot of flavor to my game.
5. (WoTC)FRCS - Honestly, I doubt I'll ever use anything from this book, but it is quite gorgeous. I can't imagine how much money it cost them to make this book. Tens of thousands of dollars to be sure.
6. (WoTC)MotP - Another great book. I got it on sale for half off, so I had to pick it up. I might never use it, though.
Um, well I'll have to stop there, since that's all the books I own. There are some things that Mystic Eye/Thunderhead Games is working on that I can't wait to get my grubby hands on, which I'm sure will go on my list, but for now that's it.