FYI not a great programmer. I've fiddled with Haskell, Ocaml, Common Lisp and Clojure. Haskell is the only language where I don't really have think about writing clean code but somehow code that comes out is pretty neat and clean. When it comes to prototyping ideas though every language seems to be on par though
Haskell is specifically designed to make easier to write pure functional code. But "cleaniness" is too subjective and I'm not sure how to measure it even very roughly.