It seems likely that biographers may have given the "favorite" status
to Susy based upon the letters Clemens wrote to his friends shortly
after her death.

On 10 Sept. 1896 to H. H. Rogers: "The family has lost its prodigy."

On 24 Sept. 1896 to Howells: "There was not a detail in Susy's make-up
that was commonplace."

On 27 Sept. 1896 to Twichell: "Susy was a rare creature; the rarest
that has been reared in Hartford in this generation."

The early loss of Susy compared to the ongoing difficulties he faced
with Jean's illness as well as dealing with a headstrong Clara are
events that would have a tendency to make later biographers rate her
adoration from her father on a higher level than the affection he
expressed overtly for the other two daughters.

Barb