I will go to a classic.
The 22 Immutable Laws of Marketing
All Ries & Jack Trout
Word of mouth marketing by Andy Sernovitz.
For a specific type of business (service, product, etc) or just general? That will really affect my answer.
"Best" is tough so I'll give the three (four) that had the biggest impact:
Sorry I couldn't pick one but these three (four) really had the biggest impact.
Some of my favorites are:
There are many but to pick one I'll have to go with Made to Stick.
Cashvertising is a really solid book about marketing/advertising.
The 22 Immutable Laws of Marketing All Ries & Jack Trout
I too loved this book even though some of the purported laws do not seems to hold, in particular the first (The Law of Leadership: It's better to be first than it is to be better). Many observers say this hasn't really been true over the past twenty years in the tech and internet sector.
Another vote for Made to Stick and 22 Immutable Laws... other books that I really enjoyed were Groundswell and Empowered (both are more focused online). My new addition to this list is Why Killer Products Don't Sell. It's a really helpful book for disruptive technology companies trying to figure out how to better sell their product (essentially by better supporting their market).