i've tried to speak to player in different ways just to know this.
some how i have the summary, CMIIW.
Highest to lowest :
Perfect - Exceptional - Superb - Excellent - Extremely Good - Very Good - Really Good - Good - Fairly Good etc till abysmail.
i thought perfect is get when player play in exceptional morale, make a good mark, and we closed the game with nice pep talks.
fiuh.. so much hard to get highest morale compare to 17.
it means we keep in touch with them every time, thats why morale is develop in 18 (in many review)
remember that praise not always the best way to grow morale, event critics has a way also