The best tool to find 'best keywords' is Google's SERP and Google Trend. And to assist you in the process, you may use "Keyword Everywhere", "Answer the Publis", 'Keyword Shitter" etc. Let me explain how-
The best place to take keyword suggestions is the search suggestions itself. But there are tons of tools that'll fetch you a bulk number of keyword suggestions from a given set of keywords/key phrases/URLs. And once you have all the suggestions in hand, you may sort them up based on Search Volumes, Competitions etc. Don't forget to check the suggested keywords that Google shows below the search results.
However, both 'Search Volume', 'Competition Scores' are arbitrary data that vary from tools to tools. So, don't solely decide on one particular tool for that.
A better option to check search volume is to use 'Google Trend'. And to check the competition, I'd better go for manual analysis. For saving some time, I'd use 'Long Tail Pro' for checking Root Referring Domains, Site Ages, Site Indexes etc.