I’ll have to say either software development or systems analysis.
Studying software development gave me great problem solving-skills that I found I can apply anywhere in life (even when writing computer programs!).
Systems analysis gave me an interesting perspective where I can look at a system and see the logic (or complete lack of it) behind it. Not that useful to me just now, but definitely interesting!
Medical Terminology even though the director of the nursing program tried to talk me out of it, calling it an “empty calorie” It helped me understand Anatomy and Physiology. My nursing instructors would come to me to “translate” when they were reading charts.
The most boring (but very useful) class I had was typing in high school.
I can still hear the teacher yelling rythmically while banging her ruler on the desk, “A S D F J K L ;”
I wish I had taken typing.
Most useful/impactful so far (there are two semesters left) has been my American Way of War class. Its an honors class and I had wandered into the honors college needing another class to round out my term. The dean of the honors college said “Goldstien’s American Way of War. Its full but I’ll call him and he’ll let you in.” He was on the phone before I could sputter out that I was a pacifist with no history background. (An honor’s histroy class with no interest in the subject, and none of the prereqs…gulp)
I loved it. It was a seminar class made up of ROTC kids, upper class history majors, and me. We read a lot of real actual sources, not textbook reading, and examined every American war, including Vietnam. I discovered that its not histroy that I don’t care for, its the brainless pap that its usually hidden in. Primary sources rock. Secondary, if you absolutely have to.
In a related note, anyone who likes to read should go find a copy of The Korean War, by Ridgeway. Excellent primary source, written on Korea, during Vietnam, the author is very knowledgeable and entertaining.
Personal typing, taken in high school. It always amazes me to see highly educated PhDs sitting in front of a multi-thousand dollar computer, pecking away with two fingers.
In college, I took a course called “Biophysical Aspects of Macromolecules” that really helped me to visualize molecules like protein and RNA as discrete units with shapes and topography of their own, and how they could interact with each other in four dimensions. That course more than any one other helped me really understand molecular biology.
Most useful was a proper mathematically based statistics and probability course. I used it over and over, doing my physics thesis, teaching cookbook stats (yuk), programming statistical methods. As well as safeguarding me from gambling 
Philosophy of science and logic were good too.
Laughing Lagomorph, I so NEED to take that course. Is anything online? Did you ahev a good textbook? (I mean Biophysical Aspects of Macromolecules, not typing…)
Business Writing (thank you Sally Susnowitz).
Your current job may not require much writing, but when you need another one–and everybody needs a new one sometime–you’ll have to write a resume. Someday you’ll write a note or two to your boss. Perhaps you’ll write a personal ad or a letter to a prospective sweetie.
Each of these could make a good impression or a bad one. Unless you dig ditches–by hand–for a living, don’t ever want a different job and are planning to marry an illiterate, you’ll want to learn to write well.
IMHO
Being able to speak well, especially in front of an audience, is also a widely useful skill, but one I learned in a hard school, not in a class.
Note that a majority of the classes that people are citing were taught by good teachers. That certainly was the case with mine. It’s hard to learn anything valuable in a class that sucks.
I have a couple.
Math class (just basic math, nothing fancy) in 8th grade. I had a teacher who made it make sense. She seemed to understand what it was I wasn’t getting, and was able to explain it so that I could get it. I never took anything beyond basic algebra and basic geometry, but I use what I learned in those classes in ways I would never have thought possible.
Freshman English, high school (honors level): Once again, a great teacher made a class that could have been a big nothing into a big something. He introduced me to actual literature and taught me not to pull punches when writing - I will never forget his analysis of one of my papers, when he said, “You’re going too easy on this guy. You know you’re right - stand up for that!” A stray bit of advice on something specific that ended up being a life lesson in general.
Senior year high school - we had a class called “Soc” which was shorthand for Sociology, Psychology and Civics - kind of a catch-all class required for every senior. Of all the weird things you’d think you’d cover in a class like that one, what I actually learned was how to balance a checkbook, how to read stock market quotes, the difference between critical thinking and parroting information, how to speak in front of people, what is meant by “circular logic,” how to outline a paper and how to read a bibliography. I honestly think this class should have been called “How to function in the world.”
My senior year in high school, the football coach wanted every player to take phys ed the last period. The players could then start practice an hour earlier. I had enough credits and took typing instead. I think I got a C in it, because I was more interested in the cute teacher, but it has turned out the most useful.
In college, I think I’d go with Histology, since I learned how to make slides of the tissue from a rabbit. 
First of all, I have to tell you I took the course in 1985 or so.
Interestingly, there were no textbooks on the market which the Prof. liked, so he gave us each what was essentially a huge package of his notes that the class was based on. He claimed he had been approached by a publishing company to write a textbook on the subject but that even his notes were only enough for five chapters or so! Well, it kept us occupied for the whole semester.
I don’t remember the guy’s name either, but I will look it up and see if he has published anything in the interim.