Ah, I already know this for my girls because I put their favourite flowers in their character profiles.
So! Susan's favourite flowers are sweet peas, although she's really not fussy. She has a soft spot for lilies as Susan is Hebrew for "lily", adores huge sunflowers, and even thinks Sprout's honking daffodils are cute.
Romilda's favourite flowers are red roses, the ones with tons of petals and a really heady scent. They're so romantic!
Draco doesn't have a favourite, but he finds certain potions ingredients more interesting than others because of all the different effects they can have when brewed differently. Unlike most people, he actually likes the bitter smell of wormwood.