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Depends what you mean by popular.
Who commands the most respect? Old guard like Ai and 4 heavenly kings, people with tons of connections like Tamaki or Haneru, and daisenpais like Sora or Mito at least within their respective agencies.
Who is most watched? Seems to be anyone entertaining/cute with quality content, probably not too much different from general audience preferences, though maybe more discerning tastes on average due to familiarity with the profession.
Who is most liked behind the scenes? Hard to say since all there is to go off of are various personal anecdotes, but any contenders I would guess fall into the following classes: the charismatic and/or thoughtful types that bring people together, the ultra chill types who can get along with anyone, or the ones who are widely considered adorable.