Best platforms for online learning

It depends on what course you are looking for because some areas might have a platform of their own.
I only use Udemy and I didn't check any other platforms, because I'm pretty happy with them. But there are a few things to be aware of on Udemy...
Free courses are most of the time not worth your time.
Never pay full price for the course. Wait for a discount.
Always watch a preview of the course, because you might find instructor annoying or poor quality of the course itself.