For me it is clearly written posts that demonstrate a concept, and provide a tangible repeatable method for the audience to both validate and replicate the approach.
EarlyRetirementNow’s safe withdrawal rate series or Paula Pant’s cap Rate post are good examples.
I don’t think I’m alone in that point of view, on my old site the two most popular posts by far (mid six figure page views for each) were:
A real life case study illustrating debt recycling at work.
A tool that let the audience run the numbers on prospective rental property investments, showing what level of financing it would self support and stress testing for various mortgage interest rates.
It definitely isn’t a “me too” post, or a round up, or paraphrasing a textbook, or an advertorial, or a listicle, or a budget/net worth round up. Instead it is something both genuinely original and of lasting value to the audience.