Productive = easy to learn, ease of use, elegant, syntactic sugar, easy debugging, decent community, enforces certain functional concepts, fast
Yeah, I'm sure having to guess the precise combination of tabs, spaces and newlines to get it to compile is real productive, I'll stick to my simple, well defined brackets
Python consists mostly of FP features, but it is multi-paradigm (as is most of the other languages on my list as well). You can program object-oriented in Common Lisp if you want.
I want to remind you that OP asked what functional language would be the most productive one, NOT which one would be the most pure.