Scala or Haskell would be my vote. You already have enough imperative stuff up on your learning wall there - learning more languages won't really teach you how

Scala or Haskell would be my vote. You already have enough imperative stuff up on your learning wall there - learning more languages won't really teach you how to think differently unless they are able to produce sufficiently different ways of describing problems and their solutions. You could probably learn ruby in a day but would it make you better for doing it? Probably not so much. But learning functional programming WILL make you better - you'll be able to take the approaches and write better code in imperative languages because you'll understand how to reduce mutability and write more declarative code using lambdas in the other languages you mention.