Rough Guide is my favorite overall. I really do find it varies tremendously by destination and try to take the time to look at reviews, publication date, and scan a few books in person before selecting. For example, the Rick Steves Czech Republic was the worst book ever, but the Rick Steves Scandanavia was pretty good. The Lonely Planet Istanbul sucked, but The Lonely Planet Antartica was great (and had no competition , and the Lonely Planet Vietnam was very helpful.

The best travel guide I personally have ever used was the Rough Guide to Egypt. Amazing.