Speech in the 7th grade, which I took to avoid the class I dreaded even more: P.E. (and avoided completely over the years through various means...I have never taken a communal shower or changed in a locker room.)
In college it was P.E. classes. (Which I chose based on my ability to endure them and avoid needing a shower afterward.)
So far, it's the Pre-Calculus class I took my Junior year of high school. I used to be really good at math, getting As, but I got an F the first quarter. Then a B, then two Cs. It was so ridiculously hard. If only I had taken AP Stats instead...
At the time, honors english in 10th grade. I got so caught up in anxiety that I turned in a blank 5 page paper, with only a cover page to make it look like I did it. The first page of the blank paper, I wrote that I was sorry for wasting your time.
Either Ancient Greek 202 (Plato's Apology) or Latin Satire (Horace/Juvenal).
I'm taking both right now. FML.
Worst non-major class - physics or AP Comparative Politics (both were in high school).
I squeaked by physics with a C. I got an A in Comp. Poli., but let me tell you, that A was fought for tooth and nail with hours of homework, essays, reading The Economist every week and watching PBS World News every day.