Definitely Python and Python-like languages. For example, IDA Pro has "IDAPython" too, and there is a Python tool for the gdb debugger called "gdb-peda" which stands for Gnu Debugger Python Exploit Development Assistance.
Other than that, learn about the system architectures like x86 and x64, which is basically the instruction sets given to the processor and what each one does. For example, here is a list of commonly used x86 instructions. But you can get them from the "official source" at Intel. Next, I recommend learning C because C has heavy control over the OS still and many times, you can simply import a library from the OS and manipulate bytes directly that way. You can often do this in Python too with various libraries, but it can be more fun and "more direct" with C itself.