A few favorites come to mind--from the section entitled Concerning The Opera:
As to the matter of popularity, be unconcerned; there is music in my opera for all sorts of persons--but none for long ears.
From Musical Pedagogics:
Herr Stein is completely daft on the subject of his daughter. She is eight years old and learns everything by heart. Something may come of her for she has talent, but not if she goes on as she is doing now; she will never acquire velocity because she purposely makes her hand heavy. She will never learn the most necessary, most difficult and principal thing in music, that is time, because from childhood she has designedly cultivated the habit of ignoring the beat.
What a fine adjudicator Mozart would have been.