number one

Concise Oxford English Dictionary © 2008 Oxford University Press:
number one informal

▶noun

• 1 oneself.

• 2 a person or thing that is foremost in an activity or area.

• 3 euphemistic urine.

• 4 a first lieutenant in the navy.

▶adjective most important; foremost.

– phrases
number one (or number two, etc.) the shortest (or next shortest, etc.) man's cropped haircut produced with electric hair clippers.