What was (or is) your favorite subject in school?
I love(d) studying dead languages. I really got in to Sanskrit. There are great teachers, great learning materials, and teaching methods that have been time-tested for hundreds of years. In other subjects I always hated memorization. But when I started studying forms and declension tables I came to appreciate its role in learning, and how a set of knowledge can quickly move from memorized but not functional, to practical and useful. It's torture while you're memorizing it, but once you have it in your brain it's not very hard to make it into something you use without thinking.
I also love the mnemonic devices. They teach little kids nursery rhymes that are actually coded grammar rules. You learn a few silly little nonsense phrases as a child, and then many years later someone points out to you that those phrases contains all of the important grammar, spelling, and pronounciation rules. How cool is that.
There's a thrill to decoding an ancient text and really understanding how the words work together at a deep level, even if it's something zillions of people have translated before you. I never got the same thrill solving a calculus problem, though I know some people do.
The same did not hold true for living languages — I studied Italian for quite a long time and it never really got its hooks in to me. I did it, and I liked it, but I didn't pour myself into it like I did Sanskrit and Tibetan. It didn't reach the right part of my brain or something.
Anything artistic. (music, arts, drawing, photography, grahic design, illustration, writing)