To be honest, even though I am a Lisp advocate, I don't necessarily think it's best suited for learning functional programming, because it is by nature multi-paradigm, and just as well suited for imperative or OO programing as functional (and is heavily used in all those ways). I think it's better to start out with something that is at least "almost" purely functional, like ML or Haskell. That way one has to program functionally. Then if one wants to get back to Lisp later, one will have a good feel for how to use its functional aspects as well as its imperative and object-oriented ones.

(BTW, people say Lisp now, not LISP. :-)