I find development "speed" to be directly correlated to happiness in side projects, as in getting things done quickly.

So with that in mind, C# with the right IDE (VS + Resharper, or Rider) is faster to write than anything else I've ever used and makes working on projects incredibly nice. Add in the solid standard libraries and the ability to make everything from websites to desktop apps in the same language and it's the best choice around, for both work and play.