It's a problem choosing which one, since there are several that are equally good, but each will carry some information not found in the others.'Rough Guide', 'Lonely Planet', and 'Insight' guides are all excellent, and contain a huge amount of info. If you want something more concise, the Dorking Kindersley (DK) guides still contain a surprising amount of good info.
I always use Lonely Planet as they don't allow people to pay for inclusion. Some are better than others but I've never had one which didn't pay for itself
My partner and I travel a lot for business and holidays - we've always bought "Lonely Planet", and found them to be invaluable, very informative and unbiased when recommending places to stay, eat etc. We also do additional research on the internet before departing.
If you are looking to buy a guide book, then we normally go to the local bookshop (Waterstones) decide which book is best and then buy it over the internet for considerably less!