Music is such a subjective thing, you could never say the above composers you mentioned Serephim were the 'best', but their mental musical capacity is certainly measurable to an extent. Consider the fact that Bach used to regularly improvise fugues. To be able to do this - even if you don't like the sound of fugues - is surely unbelievably impressive by anybodies standards. Unless there are people who can improvise 2 seperate fugues in different keys at once
?