What do you mean by 'vanilla ML'? I wasn't sure there were any really around... Ocaml has tons of stuff added, and SML also has lots of added extensions.
OCaml would be fine. It's a good language with a very efficient run-time.
I'm arguing that you could throw out most of the advanced features and that would be enough for most people. Its best features are the core ones. That's why I don't really care which specific flavor of ML you pick. Any would be a big win over many languages in wide use today.
I still think some kind of vanilla ML would suit a lot of people very well.