Google adwords keyword research tool and wordtracker in my opinion. I use these two and I'm satisfied.
There are lot of tools for research keyword, but I would like to suggest you https://adwords.google.com/.
I vote for this. Google Adwords Research Tool Wordtracker’s Free Tool Google Suggest One more popular: Micro Niche Finder
All of these tools are skewed one way or the other. I believe Google's has the most data, so I always stick to this one but always take some caveats....
Google keyword research tool is great, but if you want to dig deeper and get more details, use it with tools like MS or MNF
Google Adwords keyword tool is stand for Paid keywords; however, you can get the good ideas about the keywords. Apart from that Wordtracker is also good one tool.
Other than using the Google Keyword Suggestion tool, you can also use the SEMrush tools for keyword suggestions. All you need to do is to sign up with them. A little bucks would do cause using it really gives you enough time to check keywords for your website, their stats, and your competitors.
I use Google Adword Tool & keywordspy both are good, but Google Adwords tool I like more to use its more trustworthy than other keyword tools.
Google's keyword tool seems to work fine for me, but they're are many payed versions that i have never tried.
There are many tools can help you, but most of them are not free, I suggest you should use Google Adwords Research Tool, it is free.