[pori]

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.