No matter which language you choose for productivity, I would start with the exercises in the Little Schemer or Little MLer. They're disarmingly simple while still challenging your brain to shift into a recursive/applicative mindset.
As for what functional language you want to end up using...well, it depends on your goals.
I'm personally partial to the family of languages which use ML-style type systems, which include OCaml, F#, and Haskell.
The core of F# and OCaml are very similar. F# has better libraries whereas OCaml has both a more powerful type system and more powerful module system.
Haskell is less straight forward than OCaml and F# and has many more esoteric features you need to learn before being considered an expert. Nonetheless, many have found the process of learning to be a mind-expanding process. Also, the Haskell community is much larger and more vibrant than what you'll find with any other statically typed functional language.
As for what functional language you want to end up using...well, it depends on your goals.
I'm personally partial to the family of languages which use ML-style type systems, which include OCaml, F#, and Haskell.
The core of F# and OCaml are very similar. F# has better libraries whereas OCaml has both a more powerful type system and more powerful module system.
Haskell is less straight forward than OCaml and F# and has many more esoteric features you need to learn before being considered an expert. Nonetheless, many have found the process of learning to be a mind-expanding process. Also, the Haskell community is much larger and more vibrant than what you'll find with any other statically typed functional language.