Mathematics
I love mathematics. I loved it so much I got a PhD in it.
My particular speciality is group theory, finite fields, and enumerative combinatorics, although I like algebra in general, and fields like graph theory, number theory, and coding theory. I've never been especially hooked on category theory (it seems a little vacuous to me) but I appreciate its power. My analysis knowledge is pretty poor, as is my stats and probability, but I'm working on it.
I also like the application of mathematics on disparate other bits of mathematics. For example, probabilistic number theory or the Monstrous Moonshine theorem. I don't think mathematicians should limit their set of tools (except by how much they can keep in their head at any one time).
I've tutored mathematics at university level for several years. I like teaching and finding ways to effectively communicate mathematics. I even try (now and then) to teach my non-mathematician friends maths. One day I'll smash the myth of "you've either got it or you don't in mathematics".