Which is the most chill department of IIT Madras?

There is no straight answer for this. It is totally subjective.

Almost every department in the college is chill. It depends on whether you are interested in the branch or not. If CS and EE were very difficult, they would not have gotten the President medal from the past few years.

Every department will look chill if you are interested in what you are studying. Anybody can say their department is chill from their perspective but you might feel different about it. IITs do not have a chill scenario. What makes your years at IITs sail smoothly, is your passion for the subject that you are pursuing.