> I imagine I'd like to learn the most widely-used

Learn yourself some Clojure for great good.

Light Table implements clojure, right?

Then I can get busy with that ahead of time (I've been sweating a Python release)!

Thank you.

> Light Table implements clojure, right?

Yep!

Happy hacking: learning some Lisp variant is a great day/month/year/lifetime in any programmer's life.

Racket is another obvious choice.

Clojure has its selling points, but if you're learning Lisp for fun or intellectual gain, I'd definitely recommend Racket.

Common Lisp is the obvious choice