number one

1. One that is first in rank, order, or importance.
2. Slang. One's own interests; oneself: watching out for number one.
3. Informal.
   1. The act of urinating.
   2. Urine.

1. First in rank, order, or importance: the number one team in the nation; our number one problem.
2. Foremost in quality; first-rate: bought some number one farmland.

noun

A leading contestant: front-runner, leader. See precede/follow.

adjective

Most important, influential, or significant: capital, cardinal, chief, first, foremost, key, leading, main, major, paramount, premier, primary, prime, principal, top. See important/unimportant.

A lieutenant junior grade in the navy or the coast guard.

See the Introduction, Abbreviations and Pronunciation for further details.

(Vietnam) The best.

The noun has 3 meanings:

Meaning #1: (informal) a reference to yourself or myself etc.

Meaning #2: the first or highest in an ordering or series
  Synonym: first

Meaning #3: the first element in a countable series
  Synonyms: first, number 1