Martin Fowler brings up a good topic on his blog : "GroovyOrJRuby"
I like the way Martin ends this post... Ruby (and Rails) have brought the dynamic (we used to call them scripting) languages back to the forefront.
There is a need for different languages, to help the programming/software engineering world grow. Finding different ways to do things. The same way we have Ruby, Python, Perl, PHP... we should have JRuby, Jython, PHP/Quercus, Groovy.
Which language is the best for the JVM? I would not jump right into saying Java... it really depends on what you are doing.