Depends on what you want to use Scheme for. Racket - wide support, ease of use, etc. GNU Guile - extending GNU applications or building your own robust ones.

Depends on what you want to use Scheme for. Racket - wide support, ease of use, etc. GNU Guile - extending GNU applications or building your own robust ones. Chibi - embedded applications. (My personal favorite.) If you mean "Lisp" as in more general than Scheme, then the main two are: Common Lisp - most used, advanced, etc. Clojure - most popular of all the lisps today, arguably. It has the largest ecosystem thanks to JVM interoperability.