Clojure. Without a doubt. It's by far the easiest and most productive Lisp around right now. You get a modern core library with great support for concurrency

Clojure. Without a doubt. It's by far the easiest and most productive Lisp around right now. You get a modern core library with great support for concurrency, and you can use Java libraries with ease. The new Clojure book published by the Pragmatic Programmer guys a pretty good read, and it gets into the important aspects of functional and concurrent programming. I've been having a blast so far.