Quick rainbow tierlist

S-rank
Dodge and barrier. Defensive staples for budget teams, these rainbows can carry you through endgame challenges with little support.

Kerria, Ionocidium, Lycoris, Pumpkin, Herbaceous Peony, Saffron, Victoria (June Bride), Serissa, Katakuri, Purslane, Freesia, Lantana (hero), Golden Lace, Azalea, Paphiopedilum, Geranium (yukata), Apricot, Habranthus, Wolfberry, Easter Cactus, Tritonia

RG: Anemone (miko), Camellia, Cyclamen, Dalmatian Bellflower, Delphinium (sports), Rainbow Rose, Toadlily, Waterlily, Tiger Lily, Tuberose, Green Bristlegrass, Hare's Ear, Heart Vine, Henna, Jumpseed, Laurentia (ninja), Leschenaultia, Rabbitear Iris, Saffron (Christmas), Spoonwood, Evening Primrose, Sedum, Sawagikyou

A-rank
Exceptional rainbows that come with either coherent damage-focused kits or strong support options, but require a S-rank rainbow or two in the team to perform at their best.

Dendrobium, Victoria, Acacia, Hollyhock, Cactus, Bush Clover, Haretail Grass, Burning Bush, Soapwort, Stewartia (Christmas), Cacao, Curcuma, Helenium (Easter), Waterlily (bride), Apple (Tanabata), Liverwort, Cattleya (idol), Oenothera, Streptocarpus, Sparaxis, Sarcandra (Christmas), Dusty Miller (Christmas), Stock, Japanese Anemone, Pygmy Waterlily, Lotus (bride), Pumpkin (swimsuit)

RG: Alstroemeria, Odontoglossum, Flowering Rush, Vinca, Madagascar Periwinkle, Pomegranate

B-rank
Standard rainbows, which typically have one remarkable ability (debuff, 1.65x skill up, attack up depending on enemy number) on an otherwise vanilla kit.

Black Baccara, Cattleya, Flowering Peach, Anemone, Daisy, Cherry, Nerine, Oncidium (bride), Mistletoe, Helenium, Viola, Ardisia, Mimosa, Snowdrop, Epidendrum, Lycoris (miko), Herbaceous Peony (Halloween), Ivy (New Year), Lotus, Sensitive Plant, Marronnier, Daisy (Halloween), Edelweiss (New Year), Lavender (Valentine's), Wheat, Abutilon (sports), Agrostemma, Reeve's Spiraea (Easter), Wabisuke

RG: Wolfsbane, Saintpaulia, Anemone (idol), Blood Iris, Cherry (bride), Ardisia (bride), Cosmos, Cyclamen (idol), Geranium, White Tulip, Tachibana, King Protea, Mirabilis, Star Lily, Pickerel Weed, Plumeria (bride), Strawberry, Statice, Squirreltail Grass, Ipheion, Cephalanthera

C-rank
As with B-rank, but with limited synergy, unfocused kits or a lack of broken abilities. Keep in mind that even C-rank rainbows are sufficient to clear all content.

Red Ginger, Ume, Sakura, Chocolate Lily, Dogwood, Pink, Maple, Christmas Rose, Wax Vine, Iberis, Broom, Viola, Ghost Weed, Dusty Miller, Loquat, Jersey Cudweed, Aizoon Stonecrop, Serruria, Susuki (swimsuit), Ixia, Sutera

RG: Azuki, Blue Lotus, Chinese Smilax, Rudbeckia (ninja), Oncidium, Easter Cactus (swimsuit), Easter Lily, Edelweiss, Rough Potato, Rosy Lily, Red Tulip, Violet, Aburana, Mountain Lily, Holly (swimsuit), Hypericum, Hinoki, Lavender, Moonvine, Morning Glory, Phalaenopsis, Pot Marigold, Purple Pansy, Quince, Straw Flower

Special cases:
"Start with solar drive" rainbows (e.g. Cattleya, Ume, Dogwood, Ardisia) are S-rank for whale ship chapter 2.
"Boost solar drive power" rainbows (e.g. Dogwood, Ardisia, Mimosa, Broom, Viola, Ghost Weed, Jersey Cudweed, Serruria) are S-rank if you can arrange a full team of 5 girls from them.
Debuff rainbows are S-rank if you have more than 2-3.
Be reminded that rarity promoting/rainbow medal picking for esports purposes is punishable by extermination to the ninth degree of kinship.