Most Helpful Guy
I'd bring back Sirius.
It's a really hard choice though.
Most Helpful Girl
Even though Snape is my favorite character in the entire series (I have a thing for the tragic misled and misunderstood hero types), bringing him back to life would only continue his misery and loneliness. I follow the school of thought that death brings peace.
I'd have to pick Dobby. This was a horrendously difficult choice, but of all the characters I can think of, Dobby was the one free of any character flaws. All the human contenders have surely done wrong at least once in their lives. His only interest was to protect and save his friends, even at the cost of his own life. After living as a slave to the Malfoy family, he deserved his freedom. He had no ulterior motives to help in the wizarding war other than the fact that he wanted to. Fred is a close second, though. Poor George.