Using the good ol' free Google Keyword Planner is great. But there is an alternate universe of Long Tail keyword phrases that live in Google's other database--namely, their "Autocomplete database." That's the database that is accessed when you're typing queries into the Google search box (notice that list of suggested keywords that pop up as you type?)
Finding the best keyword research tool also depends on your own experience with SEO subjects such as buying keywords, long tail content, long tail SEO and using low competition keywords. For instance, a newbie looking to use long tail marketing may find something like Long Tail Pro V3 a little too confusing and a bit expensive as it’s a monthly fee and has a confusing interface.