The best I think!!
I started using Udemy few months ago. I found it much more useful than edx for many reasons: 1- the courses are much cheaper. 2- you have a life access to the courses. 3- you can start your course at any time you like. 4- the instructors are very responsive and continually upgrade their courses. The only disadvantage I found in Udemy is the variety of the courses. What I mean is in some studying field you can find only few courses about a certain topic.However,I think it is going to improve.
I’ve been wanting to learn some basic PHP for a long time since it would help me in my job. I decided to take a course on udemy and it was really great. For $20 I learned the basics and it even got me going into learning more on my own. The course was well organized, very happy with the overall experience.