Definitely Kotlin. I had a meeting with my team and talked about the advantages of Kotlin and why I believe we should start writing a proportion of the new modules in Kotlin and have asked for their input. This is one of the cases where it needs to be unanimous decision, but so far everyone seems in favour of it and looks like we'll be writing a good amount of Kotlin in the coming months.
The reason why I believe Kotlin is a great way ahead is - Great java interop, removes a huge amount of boilerplate, results in very readable code and extremely easy to learn (one of the reasons we could not switch to Scala was the steep learning curve which would be a big problem for new developers joining us).