Opinions will vary from person to person, so one film would be hard to pick. Some of his best:"Suspicion" (1941)"Destination Tokyo" (1943)"Arsenic and Old Lace" (1944)"Notorious" (1946)"Mr. Blandings...
'Wikipedia' lists seventy three feature films plus six shorts. Mr. Grant appears in one of the full length films for less than half a minute. Wouldn't that make them rather seventy two?