There seems to be an infinite number of SEO tools out there, and many for each purpose. Here are some of my favorites:

SEO PowerSuite

This product belongs in the robust, full capability category and is a good selection for small businesses and digital marketing agencies. A big plus point is that you get a lot of capability for the price. At the same time, if you are an agency, you need the enterprise version.
Right now, they have an amazing Christmas promotion at 70% off!
https://www.link-assistant.com/christmas-2017.html

Here is their vendor page link:
SEO PowerSuite Professional

Market Samurai

 

 

I’ve owned this product for over 10 years. It just keeps working and is a one-time low cost. It’s not a full-blown suite of SEO tools, but is still great for basic keyword research, competitor ranking, and domain exploration.