Answer: serenest

AnswerCrossword Clue
SERENESTCalmest.
serenestSERENE, calm, tranquil
serenestgust used as part of a title His Serene Highness
serenestclear and free of storms or unpleasant change serene skies
SERENESTLeast flustered
SERENESTMost peaceful
SERENESTLeast ruffled
SERENESTLeast disturbed
SERENESTMarked by greatest tranquility.
SERENESTMost calm
SERENESTMost placid.
SERENESTUnruffled to the last degree.
SERENESTLeast turbulent.
SERENESTCollected to the extreme
SERENESTLeast cloudy
SERENESTMost composed
SERENESTMost tranquil