And the best JVM replacement language for Java is: Java?
Java is more than a little heavy on ceremony however, and I would like a really agile language for the JVM. I have used Clojure a lot in the last year for work on one customer's project but at least for now the lack of concise and useful runtime error backtraces kills some of the joy of using Clojure. Really nice language and community however, and I expect in a few years Clojure may be my primary JVM language. I love coding in Ruby and the JRuby developers do a great job moving the sub-platform forward. However, except for large Rails applications, I don't see myself writing very large applications in Ruby: for me Ruby is a scripting language for getting stuff done quickly and easily. I do like Scala but the learning curve is steep and that means that it is difficult to find pre-trained highly skill Scala developers.
Java has the sweet spot of lots of great tools and a rock solid infrastructure. So, how to make Java more agile? I do a few things that help: I use public attributes so I don't bother with getters/setters anymore unless I am using a framework that needs them for introspection. I very much like JPA, but I am growing less fond of the rest of the Java EE 6 stack - really a lot of layers between designing and writing code and runtime; too much abstraction for my tastes. The Play! Framework is great, in general, and I am using it on my personal project and I look forward to seeing how Play! develops as an agile platform over the next few years.