In terms of current job openings probably R is best. Julia got its first stable release in August 2018, meaning that more companies could be adopting it now, but R has been around since 1995. If you like Python, Julia is probably easy to pick up though. Octave is incredibly slow and no competition for Matlab, Matlab is terrible as a programming language if you're used to Python, so it's likely you wouldn't enjoy your job if it involved Matlab.