I don't know if they're the most popular, but they're the most popular with me:

Free Spirit (I would also include Jack Flagg, but he's dead (I think))
Living Lightning
Sabra
Rage
Mastermind Excello (I would include the original Black Widow (not Natasha, the real original Black Widow) and the Phantom Reporter, who work for him, but mainly Mastermind Excello)