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Learn more than one. I would suggest, in no particular order, at least one from each of these three groups:

Static, strict-by-default ML derivatives (Standard ML, F#, Ocaml, AliceML, etc.)

Static, lazy-by-default ML-inspired (Haskell, probably others, but, really, Haskell).

Dynamic Lisp-family languages (Racket, Scheme, Common Lisp, Clojure)

I would add a Logic Programming language like Prolog to the list.

Isn't Prolog considered a "declarative" language?

Functional and declarative aren't mutually exclusive -- in fact, functional languages are generally considered to be declarative.

Yes, but it complements functional programming.

Plus it helps to understand how those logic programming libraries, like mini-Kanren, core.logic among others work.

How about w.r.t. the 'biggest bang for the buck'?

Somewhat of an erlang fanboy but...

Erlang!

For a practical language I'd pick either F# or Clojure, depending on whether you like static or dynamic typing.