1) SEMrush - I believe that among all the 3rd party software, SEMrush has the largest keyword database. Their search volume data is pretty accurate and aligns with the Google keyword planner. Also, based on the type of content that needs to be produced (i.e. informational, transactional, etc.), one can utilize different filtering options available in it.
Keyword Overview looks at the entered term on its own and addresses expected searches per month, the difficulty of competing against Page 1 results for the same term, a rough estimate of organic click-through opportunities (versus paid ads and non-organic positions on the search engine results page), and the perceived priority that the user should assign to optimization for this term, where the higher the score, the higher the demand and a lowered level of competition.
This is one of the best articles on Keyword research tool that I have encountered. However, To add more to this, I just wanna point out a tool (LSI Graph) for an outstanding long tail & semantic keyword research. After the recent Google algorithm, Google is now, however focussing more on context & semantics than keyword alone. What do you think about it?
While this one isn’t necessarily a keyword research tool, it will give you valuable insight into how the keywords you’ve been optimizing for so far are actually performing for you. You might realize that you could start ranking for more keywords with more difficult competition, for example, or that only keywords on certain subjects are working for you.
However, KWFinder has a couple of areas where Longtail Pro outshines it. Firstly, I like the fact LTP provides you with a suggested keyword competitiveness to target (as described above). More significantly, I don’t like the way that KWFinder restricts your number of keyword suggestions if you choose their cheaper pricing tier – it feels like they’re holding back some of the power of the software unless you pay more. With LongTailPro, you can see as many keyword suggestions you like so long as you’re within your monthly allowance.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).