Superlative form of sincere: most sincere.(adjective)
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Use "sincerest" in a sentence
"Turkish leaders who reviled and mistreated him and took him to court repeatedly on trumped up charges of "Insulting Turkishness" are now expressing their "sincerest" condolences to his family."
"I always like to say "Immigration is the sincerest form of flattery""
""I always like to say" Immigration is the sincerest form of flattery """