Machine Learning. I thought it was a really tough class and took more time than they say (they say workload: 5-7 hours/week -- maybe if you are perfect and your code never has bugs that you need to spend time debugging -- the course is based on programming assignments in Octave where you have to demonstrate mastery of machine learning concepts), but I put in a lot of extra time, mastered everything, finished with a 100. Andrew Ng is a top-notch teacher, even though his speaking style is very low-key. https://www.coursera.org/course/ml