One of the coolest courses has got to be African Drum and Dance Ensemble. I've never taken it, but I've been to their concerts and they're amazing! It's taught by a Ugandan PhD student, and they sing, learn dances, and drum and other instrument parts to traditional African music. They even dress up in traditional garb, and the dancing is so intense that each class is a workout!
In terms of which classes the most amount of kids take, I'm not completely sure, but I know of some that a lot of kids take. These include Intro to Psychology, Calculus I, Biology I, Chemistry I, Seminar in Composition (introductory english class), Intro to Statistics and Physics I. This is mostly because these are required for a lot of majors and as prerequisites to a lot of classes. In terms of what classes are popular because kids enjoy them, I've heard a lot of good things about Intro to Neuroscience and the neuroscience classes in general. For organic chemistry, Dr. Bandik is considered to be exceptional, so his class is popular. Besides that, I can't really think of any classes that are popular because kids like them. I suspect the ones kids like are upper level core and elective classes in their major. Regarding the honors classes, I haven't heard too many good things about them. From what I've heard, they expect you to have prerequisite knowledge that you aren't informed that you should have. In general, they just teach you random things without giving you the necessary background information. For example, I took Honors 1-Variable Calculus my first semester, which is supposed to cover calculus 1, calculus 2, and more. It covered the "more" but didn't cover calculus 1 and 2. Thus kids (like me) who were expecting to learn calculus 1 and 2 who haven't learned it yet didn't get that information from that class (however, a lot of the kids in the class took AP calculus BC, ad thus had this knowledge already). Another thing regarding the honors courses, I've heard that a lot of the time the professors will make it so most kids get a B or higher. Also, the professors who teach the honors courses tend to be better then average. My main point is that the honors courses aren't too popular, but are available if you have a good background in the subject and want to learn more in depth information.