One in seven billion
A better way to say "one in a million" because there is around seven billion people in the world and you believe that he/she is the one for you.
May: How much do you really love me?
John: I love you very much?
May: Do you think I am your one in a million?
John: No, you're my one in seven billion. Will you marry me? -shows ring-
May: Yes!
Then they got married and had kids.
John: I love you very much?
May: Do you think I am your one in a million?
John: No, you're my one in seven billion. Will you marry me? -shows ring-
May: Yes!
Then they got married and had kids.