As comments/answers indicate, some native speakers aren't particularly happy with OP's much example. That's a matter of personal preference, not grammar as such, though as this chart suggests, much the more and much the most are declining usages:
On the other hand, as this chart shows, "by far" is by far the most popular choice, and always was:
So, both OP's examples are grammatically valid taken in isolation (though #2 is definitely the more common phrasing). But as a pair they're nonsensical, since only one of the girls can be the most imaginative, regardless of whether we include "modifiers of degree" to indicate that their powers of imagination are significantly (as opposed to perhaps just marginally) higher than others.
If we're to understand the question as meaning which form conveys stronger conviction on the part of the speaker, I think it's really impossible to answer. As explained above, most people wouldn't use the first version anyway (though it could be strengthened even further by using very much). But they're all just variant ways of saying the same thing; they can't be ranked by "degree".