Although in some cases it depends on the destination, and it's always worth viewing several guidebooks side by side or reading reviews (e.g. on Amazon), for the particular destination you're going to, there are some general reasons to pick one series of guides over another. From all the series listed below I've used several different guidebooks.

Lonely Planet: I used to use these all the time, but not anymore - I think their "attitude" has become a bit forced and the new design is okay, but only looks nice because of a pervasive lack of detailed information. I think I also happened to hit a few spots where their information wasn't as up to date as I was used to also, but that's probably chance.
Rough Guides: For most places these are now my favorite. They have lots of accurate information and are just opinionated enough.
Moon: generally much weaker than the competition.
Time Out: pretty good city guides.
Rick Steves: the first time I went to Europe I used his Through the Back Door book, which has lots of decent general travel advice and some destination stuff also. But now I think he's a little annoying and his style doesn't really fit for me. But it's definitely worth looking at at least once, even an old version from a library or something.

I'm haven't used Let's Go while traveling, but I have looked at them - and not been too impressed. They're probably better suited for students - but even when I was a student I liked Lonely Planet much better.

As far as maps, I don't mind buying a separate map if necessary, since it's usually possible to find maps that are both much better and easier to carry in a pocket and quickly consult than those found in any of the above guidebooks, sometimes even for free.

Don't skimp on a guidebook, and don't be afraid to rip out and bring only those sections you need.