Depends on what you're doing. Right tool for the job and all that. C, C++, Java and PHP are popular (according to the Tiobe index). The last 3 can teach you so

Depends on what you're doing. Right tool for the job and all that. C, C++, Java and PHP are popular (according to the Tiobe index). The last 3 can teach you some OOP if you're not already familiar with it. If you're interested in functional programming, you can learn some stuff from Lisp, F#, OCaml, Haskell