Probably Scala (although I hope Kotlin makes some inroads too), but I don't think it will ever displace Java in a significant way. Scala, outliers aside, is well loved by it's users [1], but not everyone wants to spend the time to learn it. Java remains popular because it's the nobody got fired for picking it default, not because it's technically superior to Kotlin, Celon etc. The Java language has become one of the poorest choices on the JVM and is showing its age when compared to the alternatives. We ditched Java for Scala for most work. It's fantastic to work with once you've learned it, but it's not a path for everyone who doesn't have the inclination to spend the time learning.

[1] http://stackoverflow.com/research/developer-survey-2015#tech...