Determining Japan’s most popular anime or manga franchise isn’t such a cut-and-dry thing, as different criteria suggest giving the crown to different series. For example, if you’re going by longevity, pirate saga One Piece has to be in the discussion, since its manga started in 1997 and its anime in 1999, with both still going strong.
However, you could also make a case that teen idol singer story Love Live! is a contender for the title, since currently no other show’s fanbase can hold a candle to the burning passion of the Love Livers, as Love Live! fans are called.
But while the main characters of both One Piece and Love Live! and friendly, forgiving, and work well in a team, the Love Livers are feeling quite a bit more animosity towards One Piece creator Eiichiro Oda for a remark he made that many interpreted as the famous manga artist throwing shade on the way Love Live! has become a pop cultural phenomenon.