worst: the 3.5 Expidition to Castle Ravenloft Adventure.
Ha!
Best: Probably the 3.5 Expedition to Castle Ravenloft campaign (adventure). I've used this book a million times for many different games and I love the style and the detail they put into some of the encounters. Probably some of the best D&D games I've run. To be fair, the layout is a bit frustrating and the typos, editing mistakes, and unfinished ending of the book make it a little infuriating. It's also a straight-up murder machine against the PCs and you really have to be careful of a TPK at any moment (which can be good or bad).
(A close second for best is 4.0's Dungeon Delve. Not particularly well written or well designed, but a fun book that makes 4.0 1-shots soooo easy. The 3.5 Player's Guide is also an extremely well-made, well-written, and well-used book.)
Worst is probably the original 4.0 Player's Handbook, not because I disliked the system, but because 1) when it was printed it was FULL of so many typos and editing errors that I couldn't believe anyone would have signed their name to it, and 2) within months (weeks? hours?) the book was worthless because they changed all the rules. If it had been a PDF that got updated, that would have been one thing, but as a physical
book that meant that it was crap. Eventually the game had so many changes the book was completely useless as reference material.
Edit: Actually, I thought of another "worst" book, although I never bought it personally (my GM did, though, and we played it): the first pre-made adventure released for 4.0, written by Bruce Cordell. (Can't remember the title.) That was one of the worst-written, dreary slog-fests I've ever experienced. Talk about showing off some of the worst aspects of 4.0! Our GM ended up scrapping a huge chunk of stuff just because it went on FOREVER with no real purpose. It also had a near-certain TPK written into one of the earlier encounters. Bleh. Terrible design.