Long Tail Pro and Market Samurai are awesome paid tools but if you can not afford it, you can choose Google keyword planner tool which is absolutely free.
I like SemRush. It's got a high monthly fee, but you can usually find a 14 day trial on the web (like this one http://www.shoutmeloud.com/semrush-coupon-free-trial-account-code.html) Whats great it will show you what competing websites are ranking their pages for and how much traffic they are getting from those keywords. Grab the top 10 competing websites in your niche and export them into spreadsheets. From there you can do all kind of things to the data to get your answer. Combine the sheets and look for the top key out of all competitors. You can join on the keywords and look for the most common ranking. You can look for keywords your missing that the competition has. The possibilities are endless.
Aside from Google keyword planner you can also considered other mediums like related active online community that is related to your brand. Analyze their discussions to get ideas on how to target your users.
You can use Keyword Planner if you can't afford paid tool. If you can, try out Long Tail Keyword. I'm currently using it, a very good tool.
best keyword means you need to decide which keyword is best for your site and also suitable and if you want to find those keyword use keyword planner tool.
keyword planner if you want a free tool. I personally used long tail pro platinum before because it has a keyword competitiveness calculator
If You want to use free cost method You can use google keyword planner. Otherwise you can use market samurai, long tail pro.
I DON"T like keyword planner. They are so limited. I have had GREAT success using Google Suggestions - the keywords that show up on the bottom of the page.
It depends on one`s preferences. I really like Google`s Keyword Planner tool. Below you can find a link where it is explained on how to use it: https://www.quicksprout.com/university/how-to-use-google-keyword-planning-tool/
you need to do research to identify the best keyword, if you are new to this choose your best and perfect competitor and analyse them using moz, href or with ranksignals