Finding the best keyword research tool depends in part on what you intend to use if for. By this I mean are you generally using paid or unpaid marketing techniques? For example if you are using Google AdWords for your marketing strategy naturally you will be using the assortment of Google tools to set up your campaigns. This includes the Google Keyword Tool, the Traffic Estimator, The Adplanner and Google Editor. If you are using optimization or other unpaid strategies to develop your keywords there are a number of other tools which should be used to uncover the search and competition statistics of the keyword.
The Google Keyword tool is a good place to start regardless if you are using paid or unpaid traffic techniques. It provides a vast and totally free list of keywords with global search volume (the key metric to start with). From this list you want to develop keywords that adequately target your niche as well as offer modest competition for your selected strategy. For unpaid strategies such as optimization you want to examine the competitors for the top ten Google listings for each keyword. Will you be able to rank in the top ten for your selected keyword? This is the question you have to answer before you can choose a keyword to focus on.
One of the best keyword research tools for the optimization strategy is Traffic Travis. It has an excellent free version and easily gives you an assessment of the difficulty of ranking for a particular keyword. Now this is one of the new breed of tools, which includes Market Samurai and Keyword Elite among others that offer a complete exam of the potential competitor sites for your selected keyword. These tools zero in on backlinks, PR, and on page optimization to give you complete assessment of your prospects of getting onto the first page results. However, Traffic Travis, besides being free, removes most of the grunt work of assessing PR, searches and backlinks. It simply tells you, in the optimization module, if the search term is easy or hard to rank for. So it is a winner for ease of use.
Now for paid traffic you have a different set of tools in some cases. Many of these tools are free up to a point. The best keyword research tool for paid traffic is arguably Keyword Spy, with Spy fu a close second. Keyword Spy can give you stats on up to about fifteen words free and then you have to pay a subscription fee. However, the cost is worth it as you can peer into the campaigns of AdWords competitors for every paid keyword you choose. Keyword Elite is a relatively new program which is different than the others because there is no subscription fee, only a one time purchase. I find this very attractive for the new marketer. Keyword Elite generally pulls the same AdWords statistics as the others so you do not lose much in the analysis.
Finally SpyFu maybe the best keyword tool of all since it combines a broad array of statistics for both unpaid and paid traffic. It even has a very short term subscription plan, as does Wordtracker, so that you can use if for a day or two at less cost. Most of the tools mentioned have some degree of free options and you can do complete research using just that, even if you need to do more drilling into each keyword to get a complete list.