What is just the outright best material out there on any topic? The most effective, straight-to-the-point material where anyone can dig in and learn something quick. Or it's just high-quality content that you can't miss if it is in your field of interest.
e.g.
HTML+CSS : https://internetingishard.com/html-and-css/
Stats + R : ISLR (introduction to statistical learning with R)
Biology : Albert's The Cell