Two classes:
1 - Biology of Aging. Caught my interest and introduced me for the first time to the scientific literature. I'll be starting a PhD program focusing on this field later this summer, so I guess something stuck with me

The most striking and effective part of this class was definitely the focus on interpreting scientific literature. We had "journal" assignments, in which we repetitively answered the same basic questions about different canonical and contemporary experiments and reviews. Once I got used to the format, it really helped hone my critical thinking skills, because the questions required us to not only summarize the articles, but also critique results, raise questions, and relate the findings to other topics we studied. Did the results make sense and agree with various aging theories, or did they call specific models into question, etc. This was all in addition to an interesting lecture, which covered the foundations of gerontology and basics of statistical data treatment.
2 - Evolution and the Nature of Scientific Inquiry. This is probably the single most important course I took in terms of it's effect on my overall comprehension of
what science is. I wish this would have been available to me earlier in my undergraduate coursework, because it shifted my mindset from "science is learning facts and tools;" to "science is
how you can use facts and tools to learn about things that nobody else has ever discovered." I think that - at least at my undergraduate institution - many science-inclined students never experience this worldview-altering "aha" moment. So many students don't "get" the value of a science education while they are obtaining it. At least for me, this class helped push beyond the superficial enjoyment of "scienc-y" things. As for the "how," I would posit that the real key was the synthesis of 1) historical examples of scientists solving problems of their times, 2) modern ramifications of those early pioneers' findings, and 3) an effort to highlight the common "process" of science and skeptical inquiry into the natural world. We talked about the creationism/evolution and climate change debates, and had classroom-wide discussions about the merits (or lack thereof) of the claims on both sides of each debate.
Okay, three classes:
3. Organic Chemistry. When the notion of producing a molecular "fingerprint" with NMR, IR or MS hit me, I was instantly enamored with processes that alter those molecules or their proportions in biological systems. Hence I ended up completing a biochemistry major. Seriously though, O-Chem is invaluable to anybody in the life sciences. Science is all about communication, and I have noticed that those who lack OChem seem to be missing a part of the common "language" in life sciences work. I think the secret in Ochem was the way my class was taught; we could meet with our professor outside of class at regular office hours (not unheard of) - but it was always held in a public coffee shop (unorthodox). So each office hours session turned into a 5-10 person problem solving workshop with our professor (no TAs). It was fantastic - he really tried to relate to us, and despite being a talented chemist, he had no problem coming down to our level to explain the basics over and over until we got it. Dedication and enthusiasm for seeing his students succeed really made the difference.