I have written this guide @ Finding a Websites Best Performing Keywords which shows you how to find the best performing keywords on your website easily via google analytics custom reporting. This is a very easy way to find the keywords that are getting you the most forum signups or shop sales. Please take a look and let me know what you think. Thanks
You can also just check Awstats if your webhost has it (most do), just scroll to the bottom of the page.
you can see it from the top of your keywords, for example the first one with highest number is your best key word.
Basic but an effective way to analyze where your converting traffic is coming from in the SERPs. Thanks mate