Which backlink is least important and why?
No- follow backlinks are the least important because search engine does not give preference for those links.
Nofollow backlinks least important. They're used to tell search engines to ignore a particular link.
No follow backlinks, because search engine ignores no follow links and they are less valuable.
Nofollow feedback is less frequent. They are also less valuable. They are used to tell search engines to ignore a particular link. Google’s official definition of the nofollow tag is: “Nofollow” provides a way for webmasters to tell search engines “Don’t follow the links on this page” or “Don’t follow this specific link.”
All backlinks are important. Of course, do-follow ones do better things for ranking, but if you'll have only do-follow links, Google will just ban you. So balance is very important.
But let's say links on pages that are not ranking are useless.