MATH. Any kind of math. I have sucked at it all my life. When we would take those standardized tests that rate how you're doing compared to other kids your age, I would always score in the 99th percentile in English/verbal, but my math scores were in the TANK. In high school I had to be in the "special" class where they split Algebra 1 into two years instead of one, and I still barely got by. I was a senior before I was able to take Geometry, and I ended up dropping it because I just couldn't hack it.
So...yeah. Math.
French class was also not a particular favorite of mine, mostly because of the public speaking involved.