For me, personally, definitely Haruhime.

Out of all the Danmachi girls, attractive wise, my top 3 would be-

1. Haruhime

2. Ais

3. Freya

Personally in the serie my top 5 are:

1- Haruhime

2-Liliruca

3-Ais or Hestia (can never decide between the two)

4-Freya/Syr

5-Aina

My point is Haruhime is first for me

It depends on the media.

In MS, despite Yasuda's terrible art, I would say Haruhime. Although maybe that's just because Ais has no art in recent vols so I judge her based on the early vols' illus.

In SO, both of them are equal, I guess. Haimura's art is nice to see, but the characters' faces look roughly the same quality.

In SO manga, it's Ais, of course. Though I really don't like SO manga art lately.

In anime, it's Haruhime.

In Danmemo, it's Haruhime.

In the compilation books (SS books, Orario Stories book), Ais looks better.

The beauty of Freya is undeniable but her recent deeds made the woman something detestable.

Personal opinions vary.

But if you're asking what's the official canonical stance, Ais is the most beautiful mortal girl in the whole damn world. That is the position of the books. Haruhime, Ryu, etc, are all gorgeous in their own right, but Ais is #1.

Ais did received over a thousand engagement propositions from many males.

Ais right now is handicapped to finding love and understanding she is obsessed in revenge like the Count of Monte Cristo.

For me it's Haruhime