My favorite subjects were physics, calculus and biology. The hardest was An-Nahu (Arabic grammar).
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My favorite classes in high school (a. k. a. Gymnasium) were history, English, German and psychology. I'm very much into humanities (Geisteswissenschaften). At Uni, we don't have different subjects like in American or British colleges. You only have the subject that you major in. Or if you do a double major like myself, you have those two subjects. In my case, I'm getting a degree in history and English linguistics and literature.
I've never really had any subject that I hated. It might sound strange but I really liked all subjects. Especially in high school I actually really enjoyed going to school and learning new things. However, I did have subjects where I wasn't very good at. Especially math was one of my weaknesses (but somehow, I strangely still liked it). At Uni, I've attended an introductory class into the Japanese language (grammar and writing system) last semester just for fun and though it was very interesting, it was also incredibly challenging.1