I was aiming for Windows/Linux/Mac/iOS/Android.
In the end the two platform that were closest at the time (summer 2016) were HTML/Javascript and C#. Or C/C++ with OpenGL for the hardcore (not me!).
The real killer these days is the GUI toolkit. Ideally you'd want a SWT/Wx type wrapper around native widgets, most likely with a desktop flavor and a mobile one. Unfortunately nobody's investing in it, I think there's not enough money to be made. And there's no OSS community for it...