a ten

a 10

Someone or something regarded as perfect or superlative in some way (as on a scale of 1 to 10). This weather is a 10 in my book—lots of sunshine with a cool breeze. A: "Wow, that girl is gorgeous." B: "I know, she's a real 10."
See also: 10
Farlex Dictionary of Idioms.

a ten

n. the highest rank on a scale of one to ten. On a scale of one to ten, this pizza’s a ten.
See also: ten
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • a 10
  • of its kind
  • bear a resemblance to
  • bear a resemblance to (someone or something)
  • be the living end
  • living end
  • living end, the
  • the living end
  • be the spit of
  • be the dead spit of (someone)
References in periodicals archive
As filtered through her fine prose, it's a moving and often scary trip, sort of a Ten Stupid Things I Did to Almost Mess Up My Life.
Promiscuities