L.A. Reid -- Britney Spears Would Have Won 'X Factor'

If "X Factor" existed before Britney Spears was famous ... and she competed on the show ... Brit would easily have won the grand prize -- this according to "X Factor" judge L.A. Reid.

Mr. Reid was at the airport in New York this afternoon, and we asked how Britney would have fared if she was on "X Factor" as a young teen -- and Reid replied, "If I was a judge ... she would have won."

But if Reid WASN'T a judge ... we gotta ask ...