Undergrad:
"The Renaissance Mind: Florence 1300-1600" -- This class was the first time I really engaged in interdisciplinary research, which kind of opened up my dramatic leap into academia. It was also the first class I took in which my professor took an interest in my work and suggested that I pursue an academic career.
Grad School is more of a toss-up:
"Early Modern Literature and Material Culture" -- Again, this was a turning point for me, as it was my first seminar I took in my interest area. The class was great, but what I valued more was how supportive the professor was/is of my work. It also didn't hurt that about 80% of the texts we read were dramatic texts, but I might be biased in that respect
"Race, Gender, and Sexuality in 18th Century Literature" -- I will admit to kind of disliking the 18th century, mostly because I lack the ability to properly enjoy the novel form. However, I learned so much theory in this course and the professor, though initially intimidating because she's been producing new scholarship steadily for the past 35+ years, has been incredibly supportive of me and my work, even when I am feeling stupid and utterly useless.