Google officially reported that they do not use Keywords meta tag at all for the web ranking.  And even description meta tag is not used for the web ranking although it is used as the snippet of the Google search result page.

http://googlewebmastercentral.blogspot.com/2009/09/google-does-not-use-keywords-meta-tag.html

Our web search (the well-known search at Google.com that hundreds of millions of people use each day) disregards keyword metatags completely. They simply don’t have any effect in our search ranking at present.

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Even though we sometimes use the description meta tag for the snippets we show, we still don’t use the description meta tag in our ranking.

According to their meta tags Google uses page, it appears that ‘title’ tag is the only important information that affects the ranking. But of course, the quality content is the most important factor.