What I really sucked at was English Literature, because you had to read a previously unknown poem and then analyse it. I couldn't do this task, was very poor at it. It took me a long time to learn to write well-structured essays - my first ones just read like a list of facts with little structure. I didn't grasp until a lot later that writing essays is not just about what or how much you know, but also how you present it. No wonder I got a D.
Also, couldn't do algebra - at least, not to advanced level. Everything else in maths (statistics, geometry, trigonometry) was ok though.