How to start fandoms wars:

Camp Half-Blood Chornicles: One word, ships.

Harry Potter: Pit Houses against each other.

Doctor Who: Ask one of two questions: "Which Doctor is the best?" Or "Which companion is/was the best?"

Sherlock: Johnlock vs Sherlloy.

Once Upon A Time: "Which character is the best?"

Maze Runner: Not quite sure... This fandom is pretty tight...

Disney: Ships.

Artemis Fowl: Also pretty tight fandom...

The Librarians: CassandraXJacob vs CassandraXEzecial