Who was the leader of the famous five?

The Famous Five are characters in a series of books written by Enid Blyton. They are siblings Julian, Dick and Anne, their tomboy cousin George (short for Georgina) and her beloved dog Tommy.
The Famous Five appear in the series of children's stories by Enid Blyton,

They are a group of young children (and their dog) who experience a series of exciting adventures.

They are Julian, Dick, Anne and George (really Georgina) and the dog Timmy.