I like Java, but more than that i like JVM itself. From my experience, Scala works just perfectly for high-load data processing and frequent calls, serving critical data.

Groovy / Grails is a great thing, as for me too. You get a JVM and a whole lot of Java Libraries accessible from Groovy code, but also dynamic typing and, more or less, lightweight framework. That's if you're already familiar with Java. I think that it's going to be pretty easy for you. There're way more things laying around, such as JRuby that runs on JVM. But that all highly depends on what you're going to develop, or what tools you're most familiar with...