I. noun
: one's own interests or welfare : oneself
he's very careful of number one
never neglects a chance to exploit a personal advantage on the theory that you've got to look out for number one
II. adjective
1. : first in rank, importance, or influence : foremost
America's number one woman golfer — Providence (R.I.) Evening Bulletin
declared stamp collecting to be “the nation's number one hobby” — H.M.Ellis
public enemy number one
2. : of highest or of high quality : a 1 2
a real number one dinner