Well beauty and fame are to a certain extent subjective.
As to if she was the most beautiful in the world, that is very much in the eye of the beholder, as far as what you find beautiful in a woman. In my experience women fall into two categories of beauty, ones that are sexually attractive and ones that are cute and adorable, with each possessing a mix of both types, in broad strokes.
Its true that her facial features wern't symmetrical, for example, look at pictures where they flip the negative, where her hair part is on the right vs the left, where it really is, something looks off in them. She didnt have the type of body to be in say Maxim or FHM, or say pose in front of a hot rod.
What she did have in abundance was a total overall package of classical beauty, long and lithe, good posture, etc. As for her face, the magic was like another poster said, she just glowed, and the best explanation I can give of it, was that more of her personality shone through that the average person.
Seeing her in person myself, i can say that the debates over photos, mags and retouching, etc are laughable as she was much more beautiful in person, by a large margin, than in TV or pictures. Also many of her pictures were taken pre-photoshop, and while ppl can go back to those and change them now, i remember all the many books I had in the early 80s of her.
Her "Ace up her sleeve" for me, at least, was that she was so adorable looking, those big blue eyes would just draw you in. She's the only woman I know of where, as a man, that her eyes could keep your attention when her cleavage was on display. She could make you feel like here was this wounded girl that you wanted to care for and make sure she was alright. Also her full lips were ahead of the Julia Roberts/Collagen Injection craze for them, and her smile would just light up like the sun, both great teeth and nice high cheekbones. Even a "defect" like her rosacea was a benefit as it was like natural blush.
Also when you look at drawings, wax figures, coins, etc. they never look right, I think its because a big part of her beauty was something intangible from within coming out.
Her beauty changed over time, the romance and engagement pics are the most adorable, with her being young and at a healthy weight, also the classic "Shy Di" haircut is the best, IMO. Though the rest of the 80s was when most women started to notice her with the "Dynasty Di" look, and while the clothes and jewels were nice, those eyes and smile were all the jewels that were necessary.
She was probably at her least pretty in the early 90s, her " mom" haircut tended to make her look older than her years, and the strain of the marriage coming apart is evident in a lot of pics.
But then, in what I would say is right up with the 80/81 pics are the Testino pics, those that say she wasnt beautiful at the end, need to see those pics again. My thought on those is that more of her inner beauty comes out in those pics,and the marriage was over and a corner turning, so those feelings would come out in the pictures.
Was she the most beautiful in the world, for me, yes. For the rest of my days I will remember the first time seeing her in 1980 on TV, blue sweater and grey skirt, and not believing what I was seeing, that a woman could be so beautiful.
There are millions of women sexy enough to induce a man to bed, but very few that can induce a man to want to drop whatever hes doing and want to care,nurture &protect them. She had that in spades, and yet, its a tragic irony that she could never find that in the suitors she chose.
Thats my take on the subject, but with all respect to the other great beauties mentioned, she was a unique beauty like no other.