Microbiology, and Anatomy and Physiology were probably the most valuable.
The one I enjoyed the most, however, was one advanced "creative writing" class I had. Our professor allowed plenty of open discussion and the assignments weren't just busy work.
For me, any class that allowed me to use biology, writing or music was a class I tried to learn more than the curriculum. That made essays in Microbiology (term papers) easy as hell and a way for me to make some extra cash from other students by..."proof reading" their papers.
The other most valuable was Calculus. He was a REAL teacher. Before college, I thought math was my weak point (due to several poor excuses for math teachers all of the way through school). He made it seem like poetry and it clicked. I think his class was the first time I figured I could learn anything if I just got the basics down.