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 beliebt among Kpop Fans and Sones. Victoria's beliebt among Hottests and WGM watchers. Amber's the most liked internationally and Sulli's most beliebt 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.