Looks like Kotlin will soon be skyrocketing to the top of the TIOBE charts.
I use Pycharm and have used other Jetbrains IDE's in the past. They make me soooo very happy to use. That is, I have tried almost every Python IDE and suped up Vim/Emacs; None of them came close to Pycharm for me. They know cross platform. Their IDE's are just as bad ass on the macOS as they are on Windows... again, for me.
They make IDE's for just about everything and I assume they are all of stellar quality. My assumption, as of about 30 minutes ago, is that Kotlin could be a great bet for cross platform development in the near, near future. Jetbrains has every reason for this language to succeed in as many places as possible. Web, Mobile and Desktop. The fact that it runs on the JVM and compiles to JS out of the box makes me wonder why I haven't tried it yet.
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...
Except all those platforms have different paradigms regarding UI, and different ideas regarding base components. Even if you could have your code run on all of them, you'd still be writing platform specific code.
I'm still a few weeks away from launching, but I'm writing a little game that I'm going to ship on iOS, Android, Windows Phone and desktop (UWP), and web (with react-native-web.) They all share 99% of the code, except for a few native modules.
I've had to do a ton of work to get all of this working, but I'm going to release all of my changes, open source libraries, and a boilerplate so that it's easy to get started.
After the initial release, I'm going to be adding support for Mac, Xbox One (UWP), Apple TV, and maybe Linux. Mac and Apple TV are both forks of iOS that share 90% of the native code. Xbox One almost works out of the box, and I just need to add gamepad support. Linux is the only platform that is missing - Canonical's Ubuntu fork has stalled and no-one seems to be working on it.
I'm also keeping an eye on Kotlin. I just found out that it might be possible to write cross-platform React Native modules using Kotlin. (I was planning to use C++ or Rust for some shared code.)