Even though both ways are correct, it depends on the context. When you say "haya visto" you have to add a period of time, something like "que haya visto HASTA AHORA" When you say "que he visto" it means in all the time.
So, for what I think you are trying to say "Es la mujer más guapa que he visto" works better. And if you change "guapa" for "bella" or "hermosa" could be a little more poetic.