[miloshh]

Definitely Haskell. May I add one more resource for learning it that worked great for me, the official "Gentle Introduction to Haskell":

http://www.haskell.org/tutorial/

For those already proficient with programming that just wnat to pick up Haskell, it's ideal.

Then I would suggest an exercise that involves manipulating abstract syntax, for example parsing and interpreting a little language. ("Build yourself a Scheme..." might be good though I didn't read it myself.) This is where the language really shines, and you don't need any of the recently popular trickery like arrows or GADTs.