I have two, I can’t decide on one, mainly because I tend to be indecisive, so I will talk about both.
The first subject would be English and that’s because you get to read and analyze novels and short stories and study the language, whether it’s directly from the book or learning the grammar rules that applies to the writing process.
My other favorite is algebra. I know, it’s strange, but I seem to enjoy figuring out how to solve for x and knowing there is a definite answer and a balance of things. It’s calming. Weird, I know.