(Copyright © 2000-2001 --
this item revised 26 Feb 2001)
Whichever one helps you, of course!
Seriously, no one book is best for everyone. If your budget allows, you should get two books, with different strengths and presentation methods. You may find you "get" author A better on some points but author B on others. Or you might want both a textbook and a review manual.
Your best bet is to go to a decent-sized bookstore or library[1] and look at what's available. Plan to spend an hour or so making your selection -- you'd waste much more time with the wrong book, not to mention the money.
First and foremost, look to see that there are plenty of exercises. Math is like carpentry or cooking: you can't really learn it by reading about it, you have to learn by doing. In math, that means working problems. Lots of them. Obviously it's best if the book you pick has solutions or at least answers for a significant number of the exercises, so that you can make sure you really understand.
Beyond that, evaluate a math book like any other instruction manual. Look at tables of contents and see which books seem to cover the topics you need. Read sections on the same topic from different books and see which "speaks" to you best.
You can always ask for help. Ask the math department at your local high school or college what they are using. If you can talk briefly with a good teacher, ask what he or she recommends. (That may not be the school's current textbook. Textbooks are not always chosen for their technical excellence. Sometimes the school is used to doing business with a particular distributor, or the professor's friend has written a new textbook.)
A practice manual for a standardized exam is probably a poor choice. The skill of taking a test, particularly a multiple-choice test, is quite different from actually learning subject matter.
The College Outline Series and the Schaum's Outlines are good supplementary books on most topics. Generally you should not use them as your only source, because their explanations tend to be too brief and compressed for most people who are learning the topic from scratch.
For teaching yourself, one of our regulars recommends books like these, which try to take a different approach from standard textbooks:
Despite all of the above, there is one best book for teaching you to work the dreaded "story problems". It is How to Solve It by G. Polya. There is no one best book for algebra, trig, or calc; but there is a best book for learning to solve word problems, and this is it.
Since textbooks go out of print quickly you should realize that if you pick a book from the library's shelves you may have to search for a copy to buy. On the other hand, used textbooks are considerably cheaper than new, and there are search engines on the Web to help you find them.