Hello,
That's a tough questions. In general in the scientific field Python is popular, so learning it definitely does not hurt. For real-time stuff C++ is also useful.
Personally I mostly work in C# (which is also the language in which I wrote KUKA|prc) which I feel very comfortable with, in large part due to Visual Studio being such as nice IDE. I also transition very easily to Java (e.g. for the KUKA iiwa) and to a certain degree to JavaScript, because the languages are quite similar.
That being said, if you want to create your own robot logic, I wouldn't look as much at the language, than at the environment you want to work in. Because having a good geometric library and fast 3D viewport is really important. On the one hand, you have got Rhino itself of course (C# and C++), but you could also look at game engines like Unity (C#) and the Unreal Engine (C++). Unreal also has got a nice visual programming environment. Or you do it all in a browser with Javascript and Three.js, if you want to go that route.
So there really isn't a "best" language, I'd just go whatever feels best for you. The next KUKA|prc (no exact ETA) should make external scripting via C# quite a bit easier as well, but we haven't decided yet by how much we can open it up to users.
Best,
Johannes