"tranquilest" in the adjective sense
1. placid, quiet, still, tranquil, smooth, unruffled
of a body of water) free from disturbance by heavy waves
"a ribbon of sand between the angry sea and the placid bay"
"the quiet waters of a lagoon"
"a lake of tranquil blue water reflecting a tranquil blue sky"
"a smooth channel crossing"
"scarcely a ripple on the still water"
"unruffled water"
2. calm, unagitated, serene, tranquil
not agitated without losing self-possession
"spoke in a calm voice"
"remained calm throughout the uproar"
"he remained serene in the midst of turbulence"
"a serene expression on her face"
"she became more tranquil"
"tranquil life in the country"
Source: WordNet® (An amazing lexical database of English)
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