Control systems is my thing. I particularly get my kicks from robust control of complex systems. Whether it's H-inf, switched systems or just a well-tuned PID, the bottom line is always: know your system.
Over the last few years I've transitioned into deep learning, particularly for computer vision.
I like to keep myself informed of what's going on in IT security (and keep my paranoid self awake at night).
credit for profile picture: https://xkcd.com/481/