The usual recommendation "for learning and to experience low level programming" would be C. See this for example http://www.joelonsoftware.com/articles/CollegeAdvice.html although that was written before Go and Rust.

Also if you're mucking about with machine learning a lot of the libraries seem to be in C or C++. I played about a bit with computer vision using Python but sometimes the Python bindings don't work or don't exist and I was thinking it would be easier to just call the things from the C/C++ they are written in.

We're actually growing a rather nice machine learning ecosystem on the Rust side, though obviously it's much less mature than the C/C++ solutions. See https://github.com/autumnai/leaf/blob/063ce978004b8bf4b7fc74... for an example.

In that case, I would recommend Rust because it's just harder to learn than the other options.