Based purely on abilities:

S:
Cyrtanthus

A:
Lavender, Holly, Queen of the Night
Edelweiss
Oncidium, Dipladenia

B:
Robinia
Toad Lily, Apple of Sodom
Snapdragon, Mirabilis, Cherry Sage, Lupine, Gentian

C:
Lantana
Waterlily, Monotropastrum

D:
the rest

F:
Iris

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Dogwood has a meh ability, but you'd be silly for passing up the opportunity to level a rainbow.

Cyrtanthus has the important 1.2x skill up ability, so she should be near your top priority.

Peach is the strongest gold, having even more TP than some rainbows. Definitely level her as well.

Convallaria is also one of the strongest golds, but she's a solar blast unit, so she may not be useful for afk setups.

Lavender's, Holly's, and Queen of the Night's heal ability will be important for endgame maps. Lantana is arguably useful as well (but don't level her with Chocolate Cosmos if you get her later, as their abilities don't stack).

Edelweiss and other anti-puny pest abilities (Robinia for heals, Toad Lily and Apple of Sodom for -50% damage) are good for maps like 12-5. If you opt for fast teams (like Oncidium and Dipladenia) instead to clear the maps, they may not be as useful, but it'd still be good to have Edelweiss leveled, at least. There's no denying that she's one of the best event girls along with Lavender. On that note, Oncidium and Dipladenia are great to have as well to balance out your slower units.

Mountain Lily is strong, but is relatively lackluster outside of her high HP count, which gives her less utility than Lavender/Edelweiss, but is otherwise on par with those two.

Snapdragon, Mirabilis, Cherry Sage, Lupine, and Gentian (ordered by strongest to weakest) are good for defensive/counterattack squads. Waterlily and Monotropastrum are for dodging. If you want a defensive squad, use whichever combination of units you like or feel works best. Do note that Waterlily and Monotropastrum have lower stats to compensate for their dodge skills, with Waterlily being the weakest gold outright.

Tachibana and Moonvine have the odd "skill up on third turn" ability. Moonvine is also quite weak, so people typically use Moonvine for her speed instead. Tachibana has okay stats to back up her odd skill.

Iris has a crappy ability, put her on low priority. Canna and Calla are infamously weak for event golds, so they are also low priority.