Well, it really depends on what your current stack is. If you're doing .Net then F#, if you're doing Java look at Closure and Scala. You can look into or Erlang Haskel as well.

I'm saying this because functional programming has the potential to simplify software architecture in some cases and make you a better developer or architect. Learning a new paradigm makes you question current practices more and that is a good thing imho. Recently I've been looking into Racket and I'd probably never do production stuff on it but it's been a real eye opener to me. I also like F#. There are a lot of resources out there, one of them is Scott Wlaschin, look him up on youtube.