Basically, Google shows the autocomplete suggestions whenever you start typing anything into Google search box. It is in Google's best interest to show the most relevant keywords in the autocomplete suggestions. Keywords that would help Google to retrieve the most relevant websites and help users find the most relevant content for their search query.
When you’re running the Rank Checker feature sometimes you see Google’s captchas pop up and which can be annoying. I have only experienced this on a computer with an old OS (Windows XP) though; I haven’t seen this on Windows 7 or Windows 10 machines. Again, if you close the software and reopen it after a few minutes, everything will be okay again. UPDATE: one of the latest updates fixed this issue.
Tip: Instead of targeting large keywords (with lots of monthly searches) in one main article – try targeting lots of long tail keywords in lots of different articles. I’ve been having success with this strategy for a long time now – and more recently Spencer and his team at Niche Pursuits have been using this strategy and talking extensively about it.
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.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.