My goals are: 1) to get the "Lisp experience" with minimum overhead; and 2) to use the best available tools.
And I hope these two goals are not completely contradictory. I want to be able to write my own application on my computer conveniently after a few minutes, and to fluently progress to more complex applications. On the other hand, if I happen to later decide that Lisp is not for me, I want to be sure it was not only because I chose the wrong tools.
Thanks for all the answers! I will probably start with Racket.
For a certain value of "the Lisp experience", Emacs may be considered more or less mandatory. In order to recommend for or against it I would need more precise knowledge of your goals.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.