Rust, Python, Javascript gives you a broad spectrum of experience and you can probably adapt most of that to 90% of the other languages you'll probably run into.

If I had to pick one of the three, it would absolutely be Python for ubiquity. I believe the thing that makes someone confident and capable in a language is writing a lot of code, and Python makes it really easy to explore a broad range of problems-domains which means you can follow whatever interests or whims you have which will hopefully encourage you to write more code.