Introduction to Mathematical Thinking ( https://www.coursera.org/learn/mathematical-thinking ). Aims to teach you what it is like to think like a mathematician. Covers the elements of topics that you probably encounter in the first semester of an undergraduate maths degree: logic, induction, proof construction, real analysis, etc.
Machine Learning ( https://www.coursera.org/learn/machine-learning ). I'm still working through this course but am finding it extremely interesting. I find that having to implement things in matlab/octave gives you a deeper understanding than using a framework like tensorflow or keras.
Both of the above courses have good instructors, which I think is the main factor that makes a good mooc.
Coursera's 'Introduction to Mathematical Thinking' is a great starter course for real Analysis https://www.coursera.org/learn/mathematical-thinking
A couple of years ago I did the Introduction to Mathematical Thinking course on Coursera . Even though I found it hard, I enjoyed it and learned a lot, and I feel I got some insight into mathematical though processes. Recommended.