While it has nothing to do with my current work, introductory digital systems was definitely one of my favorite classes. It was amazing to go from logic gates to adders and muxes and whatnot, to state machines, flip-flops, and useful computation. Now I work in software, and although it doesn't really come up, it's very satisfying to understand down to first principles how computers can be built up from the gate level to whatever ridiculous level of abstraction we work with on a daily basis. That's what I love about engineering generally, the ability to roughly understand what is going on around me down to some first principles.