Most diggity describes the "best in class" "or best of the best" of anything considered to be "diggity". Diggity is a term which enhances a person or thing, e.g., no diggity = no doubt, or, bomb diggity = better than the bomb (bomb here translates to great in the modern vernacular)... so, bomb diggity is better than great. Mostdiggity is better than the best which can only be described by the limits of one's imagination, and that it is "the one" entity in every category or type that cannot be exceeded, thus the words are not separated.
There were a bunch of stand up comedians at the show. While they were all funny af, Richard Pryor was the most diggity. (as in the funniest of the funny). In fact, as far as comedians are concerned, many think of him as mostdiggity among them.