Someone posted the most recent rainbow tier list from 2ch.

SS: Ghost Spurge, Black Baccara
S: Broom, Cherry, Daisy, Viola (yukata)
A: Ume/Mei Plumblossom, Sakura, Saffron, Nerine, Red Ginger/Alpinia
B: Acacia, Iberis, Epidendrum (swimsuit), Oncidium (bride), Snowdrop, Dendrobium, Euonymus, Serissa, Lycoris (miko), Maple, Lespedeza
C: Apricot, Ionocidium, Lagurus, Victoria, Victoria (bride), Katakuri, Cattleya, Christmas Rose, Fritillaria, Garden Peony, Garden Peony (Halloween), Soapwort, Hoya, Jacobaea, Hollyhock, Dianthus, Habranthus, Viola, Lycoris, Loquat, Pumpkin, Ardisia, Mimosa, Mistletoe, Kerria
D: Anemone, Cactus, Dogwood, Flowering Peach, Helenium