Scheme is probably the "purest" Lisp around, but unfortunately is not very big (it's more of a learning language than a production one). Personally I would rec

Scheme is probably the "purest" Lisp around, but unfortunately is not very big (it's more of a learning language than a production one). Personally I would recommend Common Lisp: it's one of the older dialects, has a solid standard definition, a fairly large community and you can easily download a lot of packages via Quicklisp. Plus, several important Lisp books (including PG's "On Lisp") use it as a reference dialect. As an implementation, SBCL is the most popular, though CLISP isn't bad either. Alternately, you could try out Clojure, this has all the advantages that come with being built on top of the Java virtual machine. I don't have any experience with GNU Guile or Racket, but I get the impression their following is still very small (especially for Guile).