In terms of raw capability, Haskell is the concurrency winner. Not only does it have basically every paradigm, they even all work together reasonably well. (Not perfectly, but reasonably well.) But I don't think you can argue Haskell is mainstream. It continues to be on a fairly slow growth trajectory from what I see, but it's not in any danger of cracking into the top tier language set anytime soon. Go just might in another 5 years.