For backend? Kotlin. Hands down. All the benefits of the mature Java ecosystem, but with a language that is vastly superior to Java, itself. You get coroutines

For backend? Kotlin. Hands down. All the benefits of the mature Java ecosystem, but with a language that is vastly superior to Java, itself. You get coroutines, threads, strong static typing, etc. Not the perfect type system, because it's still limited by the JVM, but much much better than Java, PHP, etc.