GSA or Scrapebox

Which one is best for building backlinks?

Both have there unique feature.
Can't compare one with other

for creating diverse backlinks definitely GSA. For many other SEO tasks (checking indexed links, checking dof0llow links, checking alive links, scraping content, scraping keywords, cleaning up and sorting lists of millions of URLs, harvesting URLs, rank tracking, yelp scraping, etc) definitely Scrapebox

I use both for backlinks but GSA is the better product for that and Scrapebox is not a link building tool as it's primary focus as it does a ton more than that as @tazarbm stated above.

With GSA you can adjust the settings to create some decent links if that is your primary goal but there is a learning curve to set it to create good links but worth the effort to learn.