If there’s only one course need to learn in computer science branch, I will recommend Operating System.
Modern OS is getting more complex due to the development of hardware. More and more features are add to OS kernel. But the foundamental concepts are similar.
Most of OS are written by C, so in order to read OS, you need to learn C programming language first.
Keep in mind, from a programmer’s perspective, an OS is an implementation of hardwares(CPU, devices).