MOZ and Ahrefs are without doubt excellent ones, but I find them very expensive, especially if you run a small-medium business.
I'm using Ubersuggest for few years, and I think it's easy to use a tool that gives a lot of value to any online marketer.
Check out Ubersuggest free tool
Ubersuggest helps you generate keyword ideas for your content marketing strategy and production.
With Ubersuggest 2.0 free keyword tool, generate an unlimited number of suggestions for free and take your content creation to the next level while increasing your website's chances of ranking against the competition.
