Simply it's the woman you love.
My wife is the most beautiful woman in the world to me!
“You don't love a woman because she is beautiful, but she is beautiful because you love her”
Too often today we asses beauty based upon certain features, or lifestyle choices making an assumption that there is a blueprint to beauty.
Beauty is defined as "the quality present in a thing or person that gives intense pleasure or deep satisfaction to the mind, whether arising from sensory manifestations (as shape, colour, sound, etc.), a meaningful design or pattern, or something else (as a personality in which high spiritual qualities are manifest)."
There are many attractive women across the world, but the most beautiful woman in the world will be unique for each person.