C is the most important language to learn in computer science. I don't use C in daily work but the ideas in C are valuable, such as pointers and memory management.
C++ is great for game development (Unreal engine) and for implementing game hacks. A nice project you can do is implementing hacks for Pwn3: Pwnie Island.
C# is frequently used in red teaming for developing anti-virus bypasses. It can also be used for virus development, which is another interesting topic that I researched a lot before working as a smart contract auditor. Moreover, C# can also be used for game development (Unity). Maybe I will try that if I have leisure time.