Agreed on Seulgi. For a long time my favourites from Red Velvet were Irene and Wendy, but around when Irene and Seulgi had their first subunit release, Seulgi completely stole Wendy's spot in my heart.
Another example that comes to mind would be Chungha. Didn't pay much attention to her when I.O.I was a thing, but she really grew on me during her solo endavours