I'd say it depends on what your background is.
If you're coming from strongly typed language, you should be comfortable with Haskell and/or Elm.
If you're coming from more dynamic languages like JavaScript or Ruby, Clojure(Script) might be more familiar.
Though there is a trick - the most familiar option might not be the best answer if you really want to feel paradigm shift, so this depends on your intentions.
I've chosen Clojure because it makes you really think in terms of data, not even about pure functions. It does not mean though it is somewhat less pure than Haskell, you can write as pure as you need. On top of that, it has great concurrency, immutability and metaprogramming (it is LISP after all) support.