since I have no skin in this battle
I'm not disagreeing with that. If we think of who is generally considered "cute", Rei definitely takes it. But having watched both idols very closely, I've seen Sullyoon do a bunch of stuff or challenges and she made it very cute in a way I think only she can pull off.
Take for example, this challenge very few idols can pull this off. I've seen quite a few of these and I think I only saw 2 that came off natural, the other were done jokingly or "laughingly"
Nayeon is notorious for doing challenges "laughingly" shes very cute in her own way but she burst out laughing in 90% of the cute challenges she does, she'll rarely ever do the challenge as it's meant to be done.
Sullyoon is very good at "freestyling" or "choreographing" her cutes. Like when she does challenges, she not only strays away from the original but also make it cute in her own way (which she's very good at). She can be cute on the spot.
I couldn't find a challenge common enough where I've seen multiple idols do. But take Overdrive for example. It's the boy group challenge and even the original is not that cutesy.
This original Overdrive, it's not exactly an overly cute.
But since it's from a boy group, this means if a girl does the challenge and takes it on, you'll have to put your own twist to it.
Sullyoon is one of those idols who can bring up the cute from the original.
Of course Rei does it best of the entire group naturally but even Rei and everyone is just following the core of the original.
Since the original boy version isn't overly cutesy, you have to come up with your own.
Most idol default to "shy cute"
Jisoo
Winter and Karina
That's just one example but anytime Sullyoon does her version of a "challenge" its always hard to find someone who does it cuter
But on the extreme end, like the very very cutesy stuff, the Japanese level of cutesy stuff that borderline cringes and 95% of idols would avoid, Rei is one of the only idols who can do completely let go of any inhibition and do them. Her lack of inhibition on some of these things make her the only idol to ever attempt some of the challenges out there.
Challenges like these
Nako isn't very "cutesy" in terms of her personality. She does aegyo well but she's more shy cute and visually cute. Her small size mean any challenge she does ends up looking cute as a result.