[adultSwim]

I would note that several SML extensions were made by adding on to existing compilers (e.g. Concurrent ML). The Haskell community has used this model to great effect (testing/refining new features as add-ons to existing compilers and then moving some of them into the main language).

Any ML would be good. The basic features of the language are the best. "Vanilla" ML would suit most people's needs (preferable replacement to many popular languages).