I don't think it was in the beginning. Tzuyu was the most popular with the general public post debut. She had a huge CF right out of the gate, and she was the center/focus of some of their group CFs. Nayeon was #1 from Sixteen true, but Tzuyu wasn't even supposed to debut if I remember. JYPE added her because of how popular she was.
LOA era? Agree. But Cheer Up made the trio stand out (Tzuyu, Nayeon and Sana), the 3 of them were hot topics. I know Nmixx just debuted, but we don't know if their comeback will bring other members to the eyes of the public, or if Sullyoon popularity will stay as a wall, so till now Jinni isn't competing with Sullyoon to the 1# spot.