OK the very best
SEO tool that I know of is a sharp and inquiring mind, but aside from that only the Google Adwords tool takes its data straight from the databases of a major search engine so I tend to place my faith there.
But when you use this tool there are various settings aimed at the way adsense works and for that reason I recommend using only the exact terms setting.
If you use this tool regularly and keep the results squirreled away you can build a pretty nice view of how your particular keywords fluctuate monthly, which can be invaluable to online merchandisers.
But when you get all those nice long lists of Keywords you have to evaluate which you can "afford" to tackle. Now by afford I mean that if you are say, a used cars site just starting out then going after high traffic terms like cars or used cars will be a very expensive, time consuming and often frustrating experience.
As far as I know this is one of those areas where there are no handy dandy software solutions so you will just have to roll up your sleeves, have a good strong cuppa at hand and investigate just how those top ten sites got where they are.