[eru]

If you pick a Lisp, pick Racket or Clojure. All else being equal, they are the most modern and pleasant variants to work with.

[baddox]

Or Emacs Lisp, if you don't have a specific project in mind and/or aren't already heavily invested in a text editor. As a programming language in and of itself, it's not a greap lisp, but it's a great practical way to learn lisp while you're customizing your text editor.

[eru]

The only thing going for Emacs Lisp is its killer application named Emacs.

As a compromise, you might want to start people on edwin (https://www.gnu.org/software/mit-scheme/documentation/mit-sc...) instead, if you want to teach them programming. But that's probably even less useful in practice.

[dobbsbob]

or blackhat it up with tinyScheme http://philosecurity.org/2009/01/12/interview-with-an-adware...