Let's go for Mei Ume Apricot-chan. I put the "Paper Only" tag because I don't have her.
I like her bloomed form, her white and blue mantle with that sparkling things remind me a bit Makoda.

Stats:
HP: Medium HP. Not super high but still good for me. It's the HP of almost every 6*.
ATK: High. she is top tier in ATK class (11th highest ATK stat of all natural rainbows, according to the wikia)
DEF: High stat too. It's less usefull for her but a high stat is a high stat, espacially when her other stats are very solid too.

There is nothing bad to say about her stats. Her worst stat is a good one. I can't say it's perfect 'cause perfection doesn't exist here but on that point, she is a very solid unit.

Skill
5.4x damage to 1 enemy. And it seems it's 38% activation with Monday buff.
It's exactly what she needs. 5.4 damage is the best multiplier in the game, shared with a few rainbows.
With her good AtK stat and her ability set, she can be a strong boss killer, if she is used with the right allies.

Abilities
Light Gauge 50% filled
Like every old rainbow since we started, it's not very good, useless, she could have better ability... Well, it's just her legacy of old rainbow, before girls became more specialized. At least it could be useful for Whale Ship.

x1.2 Skill Activation
Good old skill activation. It's not super awesome but it's always welcome. And it help her with her good skill. A good ability both for her and for her team.

+22% ATK for all
Here too, nothing exceptionnal but still good to take.

Crit rate +20% & Crit Damages + 30%
There're her bests assets. With that, she can be a solid girl for Crit Team. Added to her beautiful skill, her skill act buff and her ATK buff, she start to be a must have for a serious Crit Team or for a bosskiller team.
She is also a good friend for Kerria due to skill act and Crit.
(sorry Wutan, I always refer to Kerria in my votes. She is my base value of broken 6*)

Conclusion
Japanese Ume Plum Apricot is a good one. Her stats are above the top, her skill is above the top and her abilities are less awesome but still super solid for a such girl. The Solar Gauge thing is meh on her but the other abilities are good and give her a place in the strongest team archetypes in the current FKG. And that's the most important.
Like I said, she is a must have boss killer and a precious unit in many teams, both for a the good support she brings and for the hard ATK power she has.
She is a nice girl. Nothing more to say.

Final verdict:
I guess A-tier is a good tier for her. She can't be S-Tier despite all her good points because she is just very strong, not broken as f*ck.
She deserve this tier for her stats and her destruction power. Even if she has a black point in her ability set. And some crazy player used her to rape Nidhogg in one turn. She can't be a bad girl.