Dev stack

I will highly recommend Kotlin. I have worked with all three intensely and so far the development speed and simplicity is the best with Kotlin. Kotlin supports coroutines out of the box. Now, it isn't something that can't be implemented in other languages but Kotlin makes it super easy to work with them. Kotlin has a bit of learning curve, so, by the time you can actually use it idiomatically chances are that you will get proficient in Java too. But once you get it, you get it, then there is no other language ;) Kotlin is backed by Google and Jetbrains team so you can expect latest programming features and good community support.