Since this is becoming a common question and people are having issues choosing which rainbow to get this week, now's a good time to break down which rainbows are good in the future.

(I have no idea why I typed all this out for you lazy bastards. Who actually chooses rainbows based off of stats instead of looks? etc.)

As a general rule, base/evolved passives only still apply if they aren't overwritten. For example, if a girl gets +10% def when evolved, but +20% when bloomed, the 20% def is what takes priority. If something isn't listed, assume their base/evolved passive still applies (like ignore movement panels passive).

For those who want the tl;dr: Dendrobium, Black Baccara, Ionocidium, Lycoris, Lagurus, Mistletoe, Saffron, and Apricot are all great choices.

* denotes new passives. Otherwise, overwritten passives will be noted in parentheses, using the notation (previously X).

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Black Baccara:
*1.2x skill chance based on affection (new)*
+65% atk on self + 1 ally (previously 35%)
*+10% more atk on party (new)*
+22% def on party (previously 15%)
+4% guard rate (previously 2.2%)

Dendrobium:
+22% atk to whole party (previously 4 units only)
*+10% more atk to self (new)*
*100% chance to attack again on first turn, 2x self skill activation also procs on the second attack (new)*

Cattleya:
*1.2x skill damage based on affection (new)*
+40% atk (3 party members including self) (previously 27%)
*+15% skill damage (new)*

Alpinia:
*1.2x~1.32x skill chance (based on skill level, not affection as usual) (new)*
+20% atk (previously 15%)
*100% counter w/ 3.75x defense power (new)*

Cactus:
+20% party atk (previously 15%)
*+15% crit (new)*
*60% chance to attack again (new)*

Mei Plumblossom:
*1.2x skill chance (all turns) (new)*
+18% party atk (previously 15%)
*+15% crit (new)*

Sakura:
*1.2x skill chance (all turns) (new)*
+18% party atk (previously 15%)
*12% more damage on raid bosses (new)*

Fritillaria:
*1.2x skill chance based on affection (new)*
+20% party attack (previously 15%)
*+15% crit (new)*
+16% self attack for every party skill used, up to 96% (previously +2%, up to 30%)

Flowering Peach:
+20% party defense (previously 15%)
+3.7% dmg reduction while guarding (previously 2.2%)
*+10% party crit (new)*
*Against bosses, +15% party atk and +8% party damage (new)*

Anemone:
*+25% party crit (new)*
100% chance to counter for 4x defense (previously 3.5x)
*+25% party atk (new)*
+22% party def (previously 15%)
+4% dmg reduction while guarding (previously 2.2%)

Dogwood:
1.2x skill chance (new)
100% solar blast gauge at start (new)
+25% attack per solar blast use, up to +75% (previously +14%, up to 42%)
+70% solar drive effectiveness (previously +40%)

Daisy:
*1.65x skill chance (first turn) (new)*
+35% atk (herself + 2 members) (previously 30%)
*-2.1% damage taken (new)*
*+10% crit rate (new)*

Kerria:
+40% atk (previously 35%)
+6.8% damage reduction (previously 3.7%)
*80%/80%/50% evasion each turn (new)*
*Increase self damage based on half of current HP / max HP (e.g. at full health, do 50% more damage) (new)*

Cherry:
*+15% party atk (new)*
*+10% more atk (3 members) on raid bosses (new)*
(if this doesn't seem like much, it's because her -15% atk on enemies was already too good)

Dianthus:
1.2x~1.32x skill chance (based on skill level, not affection as usual)
*Against bosses, +20% party atk (stacks with previous +20% attack based on affection) (new)*

Ionocidium:
*1.2x skill chance based on affection (new)*
+18% atk (previously 15%)
*80%/80%/50% evasion each turn (new)*
*40% chance to attack again (new)*

Nerine:
*1.32x skill chance (based on skill level, not affection as usual) (new)*
+25% atk (previously 15%)
*+22% def (new)*
*+4% dmg reduction while guarding (new)*

Victoria:
(bloom announced, no info yet)

Lycoris:
*1.2x skill chance based on affection (new)*
*+15% more atk for every enemy in battle (new)*

Oncidium Bride:
(no bloom yet)

Lagurus:
*100% chance to attack twice on first turn (new)*
*+120% damage based on party's movement speed (new)*

Helenium:
(bloom announced, no info yet)

Maple:
(no bloom yet)

Pumpkin:
*1.2x skill chance based on affection (new)*
+18% atk (previously 15%)
*80%/80%/50% evasion each turn (new)*
*Three party members (including self) can deal 1.5x damage to enemies weak to magic (?) in addition to their type (new)*

Viola:
*1.65x skill chance (first turn) (new)*
*+3.7% def reduction (new)*
*Three party members (including self) can deal 1.5x damage to enemies weak to pierce (new)*

Mistletoe:
*-15% atk/skill rate for enemies (new)*
+22% atk (previously 15%)
100% counter w/ 3.75x defense (previously 3.5x)

Christmas Rose:
(no bloom yet)

Ardisia:
(no bloom yet)

Herbaceous Peony:
(no bloom yet)

Acacia:
*+10% crit to party (up to 80%) (new)*
*+40% more crit to self (up to 80%) (new)*
(same thing as Cherry, she already has good base/evolved passives)

Wax Vine:
(no bloom yet)

Burning Bush:
*-20% enemy skill activation (new)*
+20% atk (three members) (previously 15%)
*Three members can deal 1.5x damage to enemies weak to magic (?) in addition to their type (new)*

Mimosa:
(no bloom yet)

Apricot:
*1.2x skill chance based on affection (new)*
+20% atk (previously 15%)
100% counter w/ 3.75x defense (previously 3.5x)
*Once per battle, negate one attack for each party member (new)*

Saffron:
*+30% def (new)*
*+5.3 dmg reduction (new)*
*100% counter w/ 4.25x defense (new)*

Snowdrop:
(no bloom yet)

Iberis:
+20% atk (previously 15%)
*+12% enemy miss rate (new)*
*Three members can deal 1.5x damage to enemies weak to hit in addition to their type (new)*

Hollyhock, Habranthus, Victoria Bride, etc.:
(no bloom yet)