Answer:

serenest

Answer	Crossword Clue
SERENEST	Most calm
SERENEST	Most tranquil
SERENEST	Most composed

SERENEST	Least cloudy
SERENEST	Collected to the extreme
SERENEST	Least turbulent.
SERENEST	Unruffled to the last degree.
SERENEST	Most placid.
SERENEST	Calmest.
SERENEST	Marked by greatest tranquility.
SERENEST	Least disturbed

SERENEST	Least ruffled
SERENEST	Most peaceful
SERENEST	Least flustered