I'm still in process finding one. I played with extempore for few days, and while it was very interesting, I found it a bit too experimental for my taste.
My next playground is SuperCollider + overtone. It immediately feels more stable and it has tons of very interesting features. I just wish to avoid learning sclang, and honestly, I'm not really attracted too much by Clojure either. Ideally, I'd write music in emacs with common lisp (using SBCL) but my options are getting very few... There is some experimental CL interface for SC called cl-collider, but it is rather experimental.
Why Lisp, why SBCL? Simply because I'm learning that at the same time, I just decided to invest into that environment while working on some other projects... It would be so nice to have these things at the same time. SoundCollider's features are so impressive, so maybe I just accept all the Clojure's quirks..