ReasonML: I don't have any qualified opinion because I haven't tried it yet.
Elm: has a great proposition but in my perspective seems a bit "stuck" at the moment. Not a lot of progress, while usually its pain points are downplayed by key people in the community.
PureScript (the "other" functional language): highly under-rated language worth taking a look. It's a bit heavier on the learning investment side, but it's not gratuitous. PS has some features that surpass Haskell (it's like Haskell done right from day one), and plenty of features that benefit people in a concrete way, like type holes (compiler tells you what's missing), row types, reflection, etc. Give it a serious try.