Short answer: C.

Long answer: depends on the task.

Longer answer: bash for scripting in general, kotlin for android, C++ 'cos i have to maintain it but it’s far from favorite, PHP if i were to do web backends. My to-learn list includes COBOL, Ada and maybe Rust (waiting for all the hype and evangelists to die down a bit). Something functional like Erland or Scala would be interesting just to fuck around with the brain a bit.