Trying to rank for the keyword “Best Headphones” will see your post, page, or article buried in the depths of a billion search engine pages, and it’s never going to surface in the top thousand pages, let alone page one…ever! But going for a keyword, or Long Tail keyword phrase “What are the Best Headphones for Sleeping” will stand you in better shape to get somewhere close, or even on, the first page.
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.
1) Ahrefs to quickly see “the big picture” when it comes to any keyword I'm researching. I can instantly see the top holders in the SERPs. I then immediately take the top holders list and go check out their sites. I need to make sure I can beat them content-wise, otherwise I will search for another keyword to try and rank for, or perhaps go down the long-tail route. The Ahrefs tool and data quality get better and better every year. It's one of my favorite tools.

I’ve owned a dozen or so Keyword Tools in my time and most of them are basically the same, but I’ve always thought the best Long Tail Keyword research tool is the very quiet and reserved Keyword Researcher Pro. I don’t see it being shouted from the rooftops and I don’t see any aggressive marketing for it, but my…it’s a real useful tool that I now wouldn’t be without.
2) SpyFu: I suggest to have paid account on SpyFu. I just need to find my competitors who using Adwords and review them using this tool. It will show me what ads and keywords they are using. Note that my competitor who paid for that particular keyword knows exactly that it is important for his business including recent trends. Also using SEO feature you can input any URL and find our which keywords they are ranking for.
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).