Why is he number one? It's simple: there is no Orlando Magic without Shaquille O'Neal.
It was seen what happened with the Charlotte Hornets after Alonzo Mourning and Larry Johnson were not able to have the success the Magic did; they were moved. It was further years down the line, but Shaq had instilled such a love for basketball in Orlando that there was no way the Commissioner was going to move Orlando.
Now, getting to actual basketball reasons; he was the main reason Orlando made the 1995 finals. The Magic had a tough road to traverse on their way to the Finals. They had to beat the Bulls and Pacers, along with a potential match-up with the Knicks. Getting out of of the Eastern Conference was no laughing matter at that time.
When Shaq entered the league, nothing like him had ever been seen. So strong, big, and agile he was like the Eighth Wonder of the world. He won not only the Rookie of the Year, but people's hearts. People were enamored with his rapping, acting and overall persona; but those couldn't hold a stick to his ball playing abilties.
He was a force, and altered many of the ways teams would be able to play their defense. His biggest supporting reason might be that many of the greatest Magic players would not be of note had it not have been for him (Nick Anderson, Dennis Scott, Scott Skiles).
As much as Dwight Howard is the face of the Magic today, we will have to see if he can ever duplicate the magic Shaq held in his palm. It will take a title or two from Dwight to ever surpass Shaq in Orlando.