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My FP experience has mainly been with Scala (with haskell on side projects).

Is OCaml the ML language de jure?

Yes, it's the most actively developed and the most featurful.

F# is another interesting ML. It has less of the features which makes Ocaml interesting but it runs on .NET so you have access to a ton of library.

SML seems more niche. I don't think it sees much use outside of academia.