The best keyword is what your site about. That just depends on your topic of discussion nothing else.
If you're trying to rank on Page 1 of Google, then the Google Adwords Keyword tool is a good place to start. Look for long-tail keywords, 3 words or more, that have 1,000 or more exact searches per month. You will find that if a keyword has thousands of searches per month, it's probably going to be too competitive to rank without a huge amount of work. Now to check the competition; a rough rule-of-thumb is to add up the Page Rank of the 10 sites on Page 1 and divide by 10. If the average is less than 3, go for it. If the average is higher, drop the keyword and look for another.