Who helps Harry Potter the most?

Severus Snape. Without the information that Snape supplies, the Order would have been toast long ago, Harry would have been killed, and Voldemort would have taken over the world. Snape protects Harry from Quirrel (and would have succeeded if Harry hadn't been stupid and run off to confront him) and from Umbridge (who wanted to use Veritaserum on him) and calls the Order to aid Harry when he ends up at the Ministry. In Harry's sixth year, Snape doesn't tell everyone that Harry is cheating. He also delivers the Sword of Gryffindor to Harry, enabling him to destroy Horcruxes. In the end, Snape's memories give Harry the information and motivation needed to confront and kill Voldemort.