A quick fyi, Sara is supposed to return by the end of the week.
I re-read your questions and realized I didn't answer accordingly.
Out of the 3 girls you mentioned, I've seen 2 so keep in mind there is a margin of error in my assessments.
Skinniest- from what I remember that would probably be Sara but there have been recent accounts of her now having a small pouch. Yoyo is skinny but she's a taller girl than Sara so she wouldn't be as tiny. Never seen Tori but I highly doubt a person who is only 4"11 would be a meatball.
Youngest- no doubt that's Yoyo, she's late 20s whereas Sara is in her 30s and reviews from guys whom I consider credible dudes here peg Tori at 35+/-
Prettiest- this category is subjective as we all have different taste and while I thought Sara was cute, I thought Yoyo was a doll face, however some members of Asian decent have disagreed with me in that aspect. Of all the things I've read about Tori, it seems to be agreed across the board that she is an attractive K girl.
I just noticed that Ph has updated their roster and their famed spinner Yoyo has returned. I've never met this Yoyo but from what I'm told she is tiny but according to Kim herself, she can be an Ice Queen.
Now this is only my preference and others may disagree with me, which is ok, but my pick is AL Yoyo.