Go ahead and down-vote me now, I'm going to criticize academia here. The question was "which courses have had the greatest impact on you?" and not "which courses have had the greatest POSITIVE impact on you?"

I am sad to say my junior year honors literature course had the most impact on my career. One of the assignments was an analysis of one or two Matthew Arnold works (Matthew Arnold's a 19th century poet & critic who's pretty well known in academia.) He's not my favourite, but not horrible by any means. I'm more of a Browning or Houseman fan, really. But in academia you study more than just the stuff you like.

Turns out my prof was a guy who was REALLY INTO ARNOLD.

I write a paper and pull some argument out of my ass; can't even remember it now. My paper came back with an F. Not because I pulled the argument out of my ass, but because it was the same argument made in an obscure journal by an obscure academic. My prof demanded to know a) why I didn't properly cite this obscure academic and b) where I had found a copy of the journal article. He apparently had the last printed copy in Texas and it was starting to deteriorate. It wasn't in our school's library, so where could I have possibly found the article? (this was before everything was on the intarwebs.)

I tried to explain I had simply pulled the argument out of my ass, and that it seemed reasonable. His counter was there was no way a mere undergraduate could have produced such a sophisticated critique of Matthew Arnold.

So I got an F on that paper and every paper I turned in afterwards. Sadly, it was past the point I could drop the course, so I wound up with an F. I dropped out of the honors program and eventually left that university.

I still pursued natural language research and still love the intersection of semantics and syntax. Fascinated by Sapir-Whorf, think Chomsky's a first class mind, &c &c

Before that class, there was a non-zero chance I would have pursued a PhD in literary criticism. Afterwards? No. F that and F academia. I still write, but not as a career. And I still enjoy reading both high-brow literature and low-brow genre. And the cool thing now is I pretty much read what I want to. I don't have a stack of "serious" books I have to read. I don't have to read everything Doris Lessig wrote. I can read another Saul Bellow novel and not feel guilty (though obviously, I'd feel pretty depressed afterwards.)

So... that one class definitely dissuaded me from a life in academia. I'm out in the real world banking coin and doing what I want.

Now... please feel free to down-vote me because I criticized academia and had the temerity to suggest I was capable of independently constructing a reasonable critique of a 19th century poet.