Lately, I would say Sulli because she's been doing a lot of activitues outside of f(x). Like, she was in "To The Beautiful You" and now she's modeling for CFs and magazines. Also, she's really young so I think a lot of people like her for that too. ^^
Krystal's 流行的 among 韩流 粉丝 and Sones. Victoria's 流行的 among Hottests and WGM watchers. Amber's the most liked internationally and Sulli's most 流行的 within drama watchers and Koreans. Luna's lacking popularity but she's boosting it with her projects and variety skill. She's also known as the most talented member all in all.