What they did
In a March 2016 live Q&A, Google's Andrey Lipattsev was asked to name the top ranking signals after RankBrain had been confirmed as the third.
What they found
He said the top two are content and links pointing to your site. That gave the industry a rare on-record ranking of the big signals: content, links, then RankBrain.
How much weight to give it
A named Google engineer stating it live, so it is genuinely on the record. It has been quoted ever since as Google confirming links and content sit at the top.
What it doesn't prove
It was an informal Q&A, not documentation, and Google later walked back the idea of a fixed top-three, saying signal importance varies by query. Treat it as a decade-old steer, not a current spec.