OK, I'll try to do this, too, in descending order of importance to me (the three Core books stand outside and above the order, of course, but I'll mention them first):
PHB - without it, you have no game.
DMG - Indispensable. This book is incredibly handy. I still am amazed from time to time when I pick it up and look at how well-organized it is.
MM - Hey, I use monsters extensively. I couldn't play without this book. Well, I could, but it'd be boring.
1. Manual of the Planes - the best bang-for-your-buck of any D&D or d20 hardback, besides the Core books. This book is - and I'm gonna emphasize this to reach the cheap seats - NOT JUST FOR EXTRAPLANAR ADVENTURES - IT IS AN INVALUABLE TOOLKIT FOR CUSTOMIZING HOW YOUR CAMPAIGN WORLD WORKS. Those who have dismissed the book because they feel it's just for planes-hopping are missing out on the potential of this book. Not only does it provide ways to make your world run in a unique way, it provides jumping off points for even more radical customization. This book could be the gateway to creating scads of new and different d20 games and worlds.
2. Epic Level Handbook - Although I actually think this book is flawed - it's too conservative (hey, give me the tools and I'll determine what's gonna unbalance my campaign), and too much space is devoted to the "epic city - it provides coverage of high-level campaigns, which is something that needs to be covered by D&D. I may not run many epic-level campaigns, but I'm glad there are guidelines for doing so.
3. Call of Cthulhu d20 - Provides a lot of material that can be used for non-Cthulhu games - classless d20, a different type of magic, firearms, etc.
4. Star Wars d20 - Provides a lot of good material useful for any scifi d20 game. It also provides an alternate magic system in the Force.
5. Dungeon/Polyedron - the adventures are a great value for the money, and the d20 minigames are invaluable.
6. Occult Lore - lots of potential here, with a lot of different magic systems. There are probably a half-dozen of them that I immediately said "I gotta use that!"
7. Oriental Adventures - all kinds of great material.
8. The Wheel of Time d20 - Provides different core classes, a different magic system, and helps clarify some things from the PHB.
9. Spycraft d20 - d20 Modern before d20 Modern. A wealth of usable material.
10. Alchemy & Herbalists - This book covers ground not covered in D&D very often. Great for really adding a lot of depth to a campaign world.
Honorable mentions - Kingdoms of Kalamar Player's Guide (good, solid material); Book of Challenges (I love this book and it's general handiness); Dragonstar (incredibly well-done), Deadlands d20 (I love the setting, and the d20 material is also very minable).
