Pick anything but Clojure, go with it, and you will be in a better position to evaluate later.
You phrased this question as which Lisp is the best medium for learning Lisp, and to me that has to include a Lisp that is built up from low level primitives, whereas large parts of Clojure are written in Java.
If you had phrased this question as which is the best Lisp to start a new project in, then Clojure may be the best choice.
