Ahrefs

Ahrefs maintains one of the largest independent backlink indexes, along with keyword difficulty scores and organic traffic estimates for any domain.

Its practical use is competitive: seeing which pages on a competitor's site earn the most search traffic, and which sites link to them, tells you where the opportunity is more directly than any keyword list.

It is expensive and has no meaningful free tier, so it suits people doing SEO as a substantial part of their work rather than occasional checks.