The Probability and Statistics course was hard for me and a big step outside my comfort zone. I cursed it a lot but I'm glad I completed it. I don't think I would have gone through with it if I hadn't payed for the certificate.
Next are Machine Learning Fundamentals, last but not least Big Data Analytics Using Spark.
I'm not really sure it made me a better (web) developer, but the process of learning keeps my brain fresh. Plus, it opens up a whole new and exciting world for me.