Kotlin
Kotlin can work extremely well with RxJava. In general, Kotlin is a cleaner and simpler language than Java, but is still compatible with Java code and Java libraries. Though it should be noted that there can sometimes be some compatibility issues with certain Java libraries and SAM ambiguity. JetBrains is aware of this issue and is looking into a fix.In the meantime, there is a helper library called RxKotlin that works around these issues with the RxJava library, in addition to giving a handful of tools.
One big plus with using Kotlin is that is supports extension functions. This in turn allows for a much cleaner implementation of custom operators, since they can be called directly from the previous operator instead of having to call compose() or lift() and passing in the custom operator.
There has been talk in the Kotlin community about implementing a pure Kotlin Rx library. With a quick look around the internet, this looks like it could be promising, though it's still a work in progress: https://github.com/badoo/Reaktive
