Absolute, first stop, go-to? Bash. I try and get what I need out of the standard set of Unix tools, piped together with some scrappy Bash scripting.
This is usually if I'm trying to write something for just me or my colleagues - as it's harder to be sure of the environment your 'program' will be running in.
If the Bash starts to get a little wild (i.e. to the point where I can't understand it easily), I'll fall back on either NodeJS or Go. Go most likely in the case where I don't want to get my head around any async issues, plus I like the standard library, but both are good.
That gets me pretty far - probably up to and including a web service.
But if it's a coding challenge I'll probably use Common Lisp or Scheme. Because it's fun!
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