I don’t think there will be a recommended web framework since that’s all personal preference and needs. Some guys from Jetbrains are working hard on GitHub - ktorio/ktor: Framework for quickly creating connected applications in Kotlin with minimal effort. This isn’t mature yet but has a lot of potential.
So if you’re looking for maturity it’s the same for Kotlin as for Java. I do feel that any framework that’s code oriented (as apposed to annotation oriented) works better in modern languages like Kotlin. So for me Spring is no option, that feels old-fashioned. I’d rather use http://sparkjava.com because I prefer the straightforward approach. It let’s you hook common template engines easily Documentation - Spark Framework: An expressive web framework for Kotlin and Java