Google Adwords has a keyword tool - I assume you're trying to improve your results in Google, so take their information about which terms have the highest search rate. You need to sign up for an account, but all serious webamasters should have an account anyway.
That is the best tool, but just doing one keyword search will not give you good results if you are doing SEO or Adwords. There is quite a bit more to it, but the final results are awesome. I recently finished a tutorial about how to use it. I do need a few reviews to see if it is clear enough. If you want a conditional review copy PM me. The condition is that you actually give me a review.
How to you define "the best" and what is your experience with keyword tools? In the wrong hands, the best tool would do nothing. But in the right hands, the worst tool can still be worth something. But IMHO, I like Google's tool because it's free (LOL) and it provides me with the data I need.