I think edx is best if you want to learn something online. I have Java and Linux certifications from edx. Also I had completed the CS50 course, so I will be getting the certificate for the same soon.
The best thing about Edx is that we can have paid as well as free certifications. Although the free certificates are not verified, but we can still get one.
They teach with the help of the video lectures and slides. And they take tests on regular basis. Also sometimes some assignments and problem sets are to be completed and submitted.
I think it is the best website for learning something online and getting a certificate for it as well.