Disqus is a ubiquitous name in the commenting space. It offers a free plan but with ads. Disqus comes with lots of extra features like viewing comments from across a site and recommended posts.
Free plan available
Ubiquitous; installed by default in many Ghost themes
Free plan comes with ads
A history of bad privacy practices
Commenters have to sign in with an external platform
$10/month: 50k views, up to 3 websites
$99/month: 150k views, up to 20 websites, advanced analytics and moderation, custom reactions