I prefer working in lower-level languages since I find making things efficient to be very interesting and rewarding. Understanding how systems work at the fundamental level is also something I try to learn. So far, I would say that C is my favourite programming language, with Python as a second for scripting. I also enjoy working with bash scripts, and love LaTeX for professional work.
Mathematics is another field that I find very rewarding to learn and work with. I am, however, a bit lacking in my skills in this field, but I am studying more abstract algebra now to help with potential future research.