Cutest or most attractive overall? I'm going to assume attractive/hot.
6*: Cattleya. From the very start, but she eludes me still.
5*: Current rivals for my top waifu of any rarity level are Leucocoryne and Alyssum. Honorable mention (and third account's Secretary slot) goes to Poppy's evolved 'dat ass' form.
4*: The lovely Cepha. My starter 4*, and still tops for her rarity. Violetta Tulip in second, Lilac in third.
3*: Priscilla Gladiolus, then Rose Diana. Early game days saw interest in Serrata Rosalba and Gomphrena Globosa, but their H-scenes soured me on them...I can't stand teases.
2*: Amaryllis, then Helia Sunflower, then Bistorta.

Take a guess.

Poppy is the cutest. To reinforce that, she decided to show off:

->Open Spoiler<-

If I was an Orthodox Mormon, Alpinia and Victoria would also be my waifus. And Lycoris. Just about any and all of Moneti's girls really.