What is the best free learning platform?
Khan Academy is the best fully free platform: everything free forever, with mastery-based practice, from a nonprofit without ads. For university-level material, the free audit routes on edX and Coursera are unbeatable.
Khan Academy is unique in this list: a nonprofit where the entire catalog is free, no premium tier, no ads. The mastery system has you practicing until you genuinely command a topic, and the range runs from elementary math through statistics, economics, and computing. For foundations, there is nothing better at any price; millions of students use it next to school for a reason.
For university-level depth, the audit model takes over: on edX nearly every course can be followed free (you pay only for the certificate and sometimes graded work), and Coursera offers the same for much of its catalog. You learn the exact same material as paying students; you just skip the paper.
Add the free tiers of the skills platforms (Codecademy and DataCamp for first steps in code and data, Brilliant's daily samples) and the honest conclusion is: money is not the barrier to learning online. The barrier is consistency, and that one is free too.