ktor is best fit for Kotlin applications. But, much like Play Framework - you can use it with just about any (or any?) JVM language. With that said, Spring Boo

ktor is best fit for Kotlin applications. But, much like Play Framework - you can use it with just about any (or any?) JVM language. With that said, Spring Boot will be vastly more mature, have a lot more resources available, bigger community, etc. There's also tons of 3rd party libraries that have support for spring and/or spring boot out of the box - such as resiliance4j, openapi-generators, etc. With that said, Spring/Boot's Kotlin support is really good. The team has clearly put a lot of effort into making Kotlin a first-class language within the Spring ecosystem. For backend work, Kotlin + Spring Boot is amazing.