Clojure would be my first choice, esp since you're fond of the JVM. Python is also good, esp for non-JVM projects.
Python and JVM languages give you access to the widest range of high-quality libraries, esp in the data processing space, and having high-quality libraries that you didn't have to write is key for rapid prototyping.
Clojure REPL-driven development enables you to define and evaluate functions inline, and the immediate feedback makes development fly. Combine this with Clojure's lack-of-verbosity, first-class Java interop that gives you access to all Java's libs, and Clojure's concurrency model, and you have a toolset ideal for rapid development that doesn't sacrifice performance.
Pedestal (http://pedestal.io/) is the Clojure Web toolkit I would use -- it's designed/backed by Cognitect (http://cognitect.com), Rich Hickey's new company.
For web applications, I think rapid prototyping requires a framework that performs a lot of magic, like Rails and Django. I don't think it depends on the language of the framework since you won't have to think of concurrency or anything like it. Rapid prototypes of web applications are concerned with request, response, and database interactions. The easier the framework makes it the better. It's not about Ruby or Python or Java.
Python and JVM languages give you access to the widest range of high-quality libraries, esp in the data processing space, and having high-quality libraries that you didn't have to write is key for rapid prototyping.
Clojure REPL-driven development enables you to define and evaluate functions inline, and the immediate feedback makes development fly. Combine this with Clojure's lack-of-verbosity, first-class Java interop that gives you access to all Java's libs, and Clojure's concurrency model, and you have a toolset ideal for rapid development that doesn't sacrifice performance.
Pedestal (http://pedestal.io/) is the Clojure Web toolkit I would use -- it's designed/backed by Cognitect (http://cognitect.com), Rich Hickey's new company.