Is StandardML a subset of OCaml and F# or does it have something they don't?
F# is the most mainstream of the 3. If you can live without the fancier type-system features (trust me, you probably can!) it's the easiest ride.
In practice you have to use a lot of C# class based code, which brings an inherent mismatch to the functional model and makes code look more like a multi-paradigm language like Scala.
The tooling and ecosystem is great (though not as good as for C#, obviously...), the language has some very nice features and design decisions, but you have to live with a somewhat messy reality.
In practice sticking to C# is probably wiser.
And a talk from Don Syme, the author of F#, on good F# code as they see it [2]. It's a great talk overall and they specifically address OOP's good and bad bits starting from about 39:00.
1: https://eiriktsarpalis.wordpress.com/2017/03/20/why-oo-matte...
When I (an utter beginner in it) code in F#, I miss ReSharper a lot. I wish there were an IDE extension with features similar that of ReSharper but for F# rather than C#.
Personally, as an Ocaml coder, I make extensive use of Ocaml's fancier type-system features and would be sad to lose them.
One could argue that not only you can, but you should (for the sake of having more readable programs).
I'm not sure there is much reason to use SML itself today, given that it has stagnated since the late 90s, but it is an excellent language for teaching programming because you don't have to sweep anything under the rug. But if you want to use it, then it does actually have well-maintained pretty good compilers. MLton probably generates faster code than OCaml and F# in many cases. The main problem with SML is that there are not a lot of SML programmers around, and hence very few libraries, particularly for handling modern protocols and formats.
It seemed like both I and a lot of other students had a lot of issues with it. It seems like the university agreed as they later moved away from it.
Basic SML might not be hard to learn but you quickly have to go to concepts like recursion that is not a very easy concept to grasp for beginners. SML seems more suited for the theoretical mathematical (for lack of better words) parts of computer science rather than the more practical parts.