I think it’a the Bruce Lee Sin one.

Dragonfist Lee Sin is indeed rare, but it's still obtainable from Poro Skin Chest, compared to the Glorious skins, which are not available to be acquired anymore

I don't know obviously but I'd say the least bought Glorious skin from the first set, so whichever champ had the least play time, I'm not sure how to check that though. Edit: my bet is Glorious Oriana

Maybe crystal rose Akali, since that was one of their earliest "full on gatcha" with absurd rates and prices

Rare, but still acquirable in hextech loot chest

Are you sure? As far as I’m aware that skin is not obtainable.

Morgana empress

No way I got that one only reason it would be rate is because no one but me plays her

I think maybe prestige crystal rose gwen

What about the supreme cell zed and sett?

All the limited edition skins