The most portable code: Fortran!
The title might not be fully correct, but pretty close to. To start with I consider here only purely compiled programming languages. I.e. no scripting (no web browser). Then probably as the C programming language is used to build most of the tools, it deserves the first position.
Then indeed, this blog is to advertise the publishing of one of my old library and fortran framework. A bit of shouting cannot harm. But, somehow, I came up with this title for few reasons. For the impatient, this is here https://github.com/mariotti/freemol.