What sweeneyrod said. Scala is also OK. It has advantage over both in some aspects, but loses in other aspects. Neither of Haskell, Scala or OCaml is _the best_ -- there just isn't the perfect language. Yet! :)

Features I care about:

* Type classes: Haskell, Scala

* Module system: OCaml, Scala

* structural subtyping / row polymorphism: OCaml

* Higher-kinded types: Haskell, Scala

Other important features, but not related to the type system:

* Powerful runtime system (multicore support, green threads, ...): Haskel, Scala (kinda, with the TypeLevel libraries, but still not 1st class)

* pure functional programming focus: Haskell, Scala (kinda, with the TypeLevel libraries, but still not 1st class)

* compiling to JavaScript (so that you can use 1 language for both backend and frontend): Scala, OCaml

... maybe PureScript would tick the most boxes ¯\_(ツ)_/¯

For the frontend, PureScript (or Reason, or OCaml with BuckleScript) work well; Rust inherits most of Haskell's type system as well.

Haskell or OCaml

I would say: Scala, Haskell or OCaml.