The world is threatened by Mephisto, the King of Minor Land, who tries to spread the Melody of Unhappiness. Mephisto is after the Legendary Score, in which the Melody of Happiness is written. In order to complete the score and protect the world, new Precure girls stand up to collect the scattered notes of the Melody of Happiness.
If you wanna check the anime rankings in Japan they are calculated weekly on http://www.videor.co.jp/data/ratedata/top10.htm#comic. Just scroll down to the subtitle that says 5.アニメ (Anime) if you didn't already know. Then you can see them, if you can't read Japanese then just put it in google search.
Thats all so One Piece and Detective Conan are the 2 most popular ongoing Anime and Suite Precure is the most popular Shojo Anime.