Instructors have the easiest time writing strong reference letters for students in their smaller-sized upper-level courses, especially participation-intensive focused on student discussion, student research, and student presentations. They find it more difficult to write strong letters for students in larger lecture-based courses.
When choosing courses, don’t just choose courses that put you in the audience. Make sure you add in smaller seminar courses, highly engaged courses, or even individually-supervised research project courses, where available.