A registry mark given by underwriters (as at Lloyd's) to ships in first-class condition. Inferior grades are indicated by A 2 and A 3. [1913 Webster]
"A 1 is also applied colloquially to other things to imply superiority; prime; first-class; first-rate." [1913 Webster]

1, 1 chronicles, 1 esdras, 1 kings, 1 maccabees, 1 samuel, atomic number 1, august 1, biosafety level 1, herpes simplex 1, january 1, july 1, may 1, november 1, number 1, world war 1