What these Google suggestions are based on is real content that lives on the web. Google is trying to connect searchers with the content they might be looking for. As a marketer, this is helpful to you because it shows you what already exists out there in the niches where you operate, and if you don’t have content on those topics yet, maybe you should.
The highest number is the one that would give you the most potential return. If you have a big-time domain and can rank pretty easily on competitive keywords, start at the top. If you’re a newer, smaller site and can’t really play with the big guns yet, it might make more sense to start in the middle of the sorted keyword research list – these aren’t the “monster” keywords in your niche, but they’re also less competitive, and therefore easier to rank on.

Of all the tools listed in this article, Moz Link explorer is an old one & quite popular.  If you want to compare backlinks between two or more domains, Open Site Explorer is worth trying. This tool works best when you have a paid account of SEOMOZ though a free version of this tool is good enough to get you started checking the backlinks of your site and the sites of your competitors.