You all realize that this is a "BEST OF SHOW" type question?
Just like a pedigree dog show, think Westminster not Miss Universe, what you are actually asking is which human female exemplifies a slowly moving target of "ideal". Another aspect that is overlooked is that in a dog show they also look at temperament, pose, obedience, etc. attributes that have nothing to do with looks.
How would I approach incredible subjective question in a practical objective way?
Well - following the dog show approach...
1) Establish recognized common breed standards. Every year the AKC allows a handful of new breeds to be "recognized". These breeds are distinct enough due to some set of unique aspects, sometimes very subtle, to require that this group of individuals be evaluated on their own merits and not those of another breed. Some of these breeds have been around for centuries, others are just getting established. I would argue that humans have not had the degree of "breeding" like dogs have had, but there are long established categories of humans that I feel we should recognize for their distinct physical attributes - again to keep this more objective. This simply avoids trying to compare a Collie to a Pug - both are fine dogs, but they diverge quickly.
2) Review in a stepped system. First, filter out the multitudes of that "breed" to get down to a large group of contenders. Does the individual pass basic aesthetic, temperament and ability standards. In the dog world missing a tooth, being jumpy or walking with a limp can quickly exclude them from consideration. Second, isolate those of close to "breed standard" to a handful to be compared. Finally, select one individual that represents the BEST OF BREED. All that remains is then to compare the various best of breed winners to pick a BEST OF SHOW dog out of this pool of winners.
3) Remember that this is a moving target. If you ran the dog contest a week later, a year, a decade you will get different results. Sometimes radically different. It still is a very subjective process. Fickle and prejudiced judges, outdated or inappropriate filtering criteria and even simple sympathy voting can sway the vote. Winning because you are wearing a pink bow is not all that logical. Over time there are traditional top picks - think of Poodles, Terriers, etc. but this is not to say others can never upset these old standard winners. I am looking at you Boston Bull.
So what does this all mean? Who is the most beautiful living woman?
Well, it has been demonstrated that if you photographically combine two women the resulting face is deemed more beautiful that any of the originals. If you combine 4 the morphed face still beats the originals. As you combine more and more you end up with a massively composted woman's appearance. Oddly the most appealing woman is actually a non-existent "average woman". The face and body proportions approach a mathematical idealized average. Just like a dog breed standard.
So who would be a good candidate to fit this averaged ideal filter?
To say it like a dog standard: The average woman is around 5'-4" tall give or take a bit. She is in her 30's and has a slightly rounded and slightly busty but still athletic build. She has brownish or hazel eyes, dark brown to black hair, light olive skin tone and overall smallish but distinct face features. She is of average intelligence is not confrontational but is alert to current tasks. She is also helpful, family focused and in general optimistic.
It is interesting that most of the women already pictured in this thread fit many, if not all, of these criteria. These are just living examples of close to "average woman" ideals.
That is why you see so many "mutt" or mixed race examples are in the running. The more you combine and average out, the more generic a person looks, the more pleasing they appear. Those with a Italian, Spanish, Brazilian, India or Romanian background where multiple peoples mixed will be strong contenders for most beautiful. That is not to say that the Irish, Japanese or Zambian are out of the running - it would just be more of a Boston Bull effect and not the Poodle winning that year.
The most exceptional woman would actually be the most average. Think about that.