number one

1 number one noun
or No. 1
1 number one
noun
or No. 1
Britannica Dictionary definition of NUMBER ONE
[noncount]
1 
a  : a person, thing, or position that is highest in rank : the most important person or thing in a group of people or things
b  : the most successful person or thing in a group of people or things
2 
: your own happiness, health, or success : yourself
3 
informal : liquid waste from the body : urine — used mainly by children or when speaking to children — compare number two
2 number one adjective
or No. 1
2 number one
adjective
or No. 1
Britannica Dictionary definition of NUMBER ONE
: highest in rank or importance