mcrow said:
Any of the ones Mike Mearls wrote are good.
Agreed. Personally, I'd recommend:
1) Wildscape: It has some nice new variants (including ranger combat styles), a weather generator (not exactly realistic, but it serves well for a game) and new rules for combat in different terrains -which I find simpler, and more complete than the ones in the DMG-. A definite plus is the "magical locations" features that can be added to regular places.
2) Portals & Planes: It also has a random plane generator, and a whole lot of different features for your plains. It also covers different portals and is overall a pretty good planar resource (I use it mixed with the MotP from WotC).
3) Dungeonscape: I got it since my dungeon-builder skills as a DM were lacking, and I don't regret it. It has comprehensive guidelines about dungeons, their functions and their inhabitants, plus (again) a dungeon generator (actually, there are 2 generators: one normal generator and one express dungeon generator). I really, really like it to this day.
4) CityWorks: And, as usual with these books, there's a city generator system (that I don't really use) plus some rules regarding urban environments. Not my favorite of the whole bunch, but a solid purchase.
Now, some others that I have, but not really use that much:
5) Monsters Handbook (PDF): It's a nice resource. As have been said, there are better out there, but still has some nice ideas for spicing up creatures.
6) Traps & Treachery I/II (PDF): Haven't really used them, so I can't comment on their trap-building rules or the content.