Learning How to learn [2]: Life changing. I wish I did it sooner.
ops-class (Operating Systems) [3]: This is by far the toughest MOOC I've taken. The Assignments are really tough. Although not impossible. Just the right amount of tough, I guess. I'm currently in the last few weeks and I've really enjoyed it every bit so far.
Interesting (Not Yet Completed): Introduction to Quantum Physics (2013) [4]: My god, I just love the teacher's enthusiasm. After few lectures, I realised I need to first brush up on classical physics before moving further (which obviously was the requirement that I ignored).
[1]: https://www.edx.org/course/cs50s-introduction-computer-scien...
[2]: https://www.coursera.org/learn/learning-how-to-learn
[3]: https://www.edx.org/course/cs50s-introduction-computer-scien...
[4]: https://www.youtube.com/playlist?list=PLUl4u3cNGP61-9PEhRogn...