The picture above is of Dumbledore's army, which forms in the 5th book. Now, without his friends Harry would have failed in defeating Voldemort, and so I think that they deserve some credit.
I think the biggest person in that (besides Hermione and Ron) was Neville. At first, Neville is that loser annoying kid that no one really wants to be friends with but they don't make fun of him either. Then, at the end of the first one, he helped Gryffindor to get those last points at the end of the year. And then I feel like he disappeared until The Goblet of Fire, when he really came into his own. Not only does he learn how to dance for the ball, but he's the one that gives Harry the Gillyweed (or however you spell it) to use in the lake. He's a quick leaner, and in the movies the actor turned out to be pretty hot. In the end, Harry couldn't have done it without Neville.
Another character is Luna. I think she helped Harry in the sense that they were both closely connected with the dead and so could see some of the same creatures that not even Harry's best friends could see. In the movie (can't remember if this was in the book), she's the one that finds him on the train beneath the invisibility cloak when Malfoy froze him because she saw these weird gnat-like things around him. I think she more proved to Harry that he's not the only one that can see and hear these things; to show him that he'snot alone with his closeness to death.
The final kids that should get credit are Fred and George. They provide the greatest comedic relief in this series, and boy did it need it near the end. They also got the students out of their OWL's when Umbridge was in charge (they were bound to leave Hogwarts at some point). They sacrificed a lot for Harry without even knowing it. They were one of the few younger adults that chose to act as one of the many Harry Potters in the beginning of the 7th book. And, in the end, George got his ear blown off and Fred gave his life for the cause that Harry was fighting for.
Harry wouldn't have survived without the adults in his life either, so they too deserve some credit.
The first and most influential was Dumbledore. He was the headmaster of the school that got Harry away from the hell that was the Dursley's and he was the one that originally wanted to keep Harry safe and thought the Dursley's was the best place to do that. He took a liking to Harry immediately (as would probably happen if you were the only one to survive a killing curse from Voldemort and happened to spend a decent amount of time getting in trouble) and new he would have to help Harry defeat Voldemort. He was a very secretive man, and we find out a good amount about him after his death that may seem slightly unsavory. But, in the end, Dumbledore was right about everything. The things he had Harry do, he knew that Harry would live up to his expectations and really helped Harry grow, even in death.
Sirius is the other character that was very influential on Harry's life. In The Prisoner of Azkaban, we're originally meant to think that it was Sirius that betrayed Harry's parents, when in fact it was Wormtail. Once we find out that Sirius is Harry's godfather, we realize how much Harry needs this vital connection with his parents. Sirius is as loving as any uncle, and even though he has to stay in hiding, he supports Harry and give him advice. Harry would have been lost without Sirius, and was lost when Sirius died at the Ministry of Magic in The Order of the Phoenix. He'd gotten too close to someone and now that person was dead. His only family left was gone, and that ultimately changed Harry.
The final adult that had the biggest impact on Harry (albeit unbeknownst to us and Harry) was Snape. That was a shocker, let me just say. We always thought that Snape was always on Voldemort's side, and it seemed completely true when, at the end of The Half-Blood Prince, he killed Dumbledore and sided with the Death Eaters. But it turns out that the whole time he was a double agent, and that he was doing everything he could to keep Harry alive and helped him in defeating Voldemort. He gave his life for Harry, and it was then that Harry finally knew the truth about Snape, that he had loved Lily Potter and promised to protect Harry.
Others that fought greatly for Harry were Hagrid (always helping Harry in every single book and seemed like one of the most real characters in this series), Lupin (also gave his life for Harry and the cause of killing Voldemort, and was also almost like an uncle though maybe more distant that Sirius), McGonagall (who, while trying to be fair to the other students and houses, did her best to help Harry and was fierce in The Deathly Hallows), Mad-Eye (I wish we would have gotten to see more of him, but he did a lot for the Order, which helped Harry), and Molly and Arthur Weasley (Arthur was almost like a father to Harry from the beginning, and Molly a mother; Harry wouldn't have made it as far without their hospitality and loyalty, and besides, Molly killed Bellatrix and that kind of speaks for itself).
Who do you think out of these helped Harry the most? Let me know in the comments!