All of these already give out the idea of what the fandoms in them are like. As there is no way to determine which fandom is the most popular one, there are several well known ones in each of them. Here are some examples:

Riverdale, The Vamipre Diaries, Harry Potter, Stranger Things ;

Fairy Tail, Naruto, newer animes like Sk8 the Infinity, Haikyuu, Bungou Stray Dogs, My Hero Academia, Demon Slayer ;

Voltron: Legendary Defender ( page owner's main fandom hi ), Avatar: The Last Airbender, The Legend of Korra, She-Ra and the Princess of Power ;

Who Made Me a Princess, I Love Yoo, My Deepest Secret ;

Ariana Grande, TikTokers like Charli D'Amelio, Loren Gray ;

Popular MCYTs and streamers like Dream, GeorgeNotFound, TommyInnit ; Corpse ;