I love your question. My favorite course was Developmental Biology.
I am a wireless engineer who studied Electrical Engineering, but not before pursuing a major I thought was going to be the right one. I started as a Biology major with the interest in becoming a Physical Therapist. It wasn't until my third year of college that I started questioning my plan and shift majors. I decided to go into Engineering, but there was still room in my schedule to continue with a couple biology classes that helped me hit my credits quota.
In Developmental Biology we spent a lot of time talking about the development of life from the one-cell stage to birth. We copied diagrams the professor drew on the whiteboard using colored pencils. We found out how the innards of us come to become what they are and how they function at the most basic level. It was just fascinating to discover such amazing intricacies... even though I no longer was going to pursue that profession. (A close second was microbiology, but that's a different story).