I think it's pretty subjective, but i go for:
sexiest - gyuri. She just has that aura, and her public image of self proclaimed goddess, well you really need to be 1 to be able to pull it off.
Cutest - seungheon. I think she's quite versatile, fulfilling all 3 criteria, but when she's at her "cutest", you'll melt. hahaha
Prettiest - Hara. Hardest of the 3 to define, and so i leave it as it is. Prettiest to me.