With dozens of platforms competing for your enrollment, "which online course platform is best" is the wrong question — the right one is "which platform is best for what I'm trying to learn." The answer changes depending on whether you want a university credential, a fast practical skill, or a cheap way to test interest in a new field.
If you want a real credential from a real university
Coursera and edX both partner directly with accredited universities, and the difference shows in the content: structured syllabi, real assignments, and in some cases actual degree or MicroMasters credit. If your goal is something you can put on a résumé next to a real institution's name, start here rather than with a marketplace platform.
If you want a specific, practical skill fast
Udemy's strength is breadth and price — individual instructors publish highly specific courses (a particular certification exam, a particular framework, a particular tool) and frequent sales make them cheap to try. The tradeoff is inconsistent quality: there's no institutional review, so check the instructor's other courses and the review count, not just the star rating, before buying.
If you're not sure yet and just want to explore
This is where free-to-audit options matter. Many Coursera and edX courses can be audited without paying, which is a genuinely underused way to test whether a subject actually interests you before committing money to a certificate track.
The comparison that actually matters
Before enrolling anywhere, compare the same topic across platforms rather than picking a platform first. Price, instructor credentials, and review volume vary enormously even for the same subject — that comparison is the entire point of a site like this one, so use it rather than defaulting to whichever platform you've already heard of.