The main protagonist is the character who is most greatly affected by the plot. It is the individual at the center of the conflict, and that who is expected to be influenced and changed by the events as they take place. It could be a hero or an antihero, as it is in this case.
In Oscar Wilde's "The Selfish Giant" the antihero protagonist is the eponymously named character of the giant. Like the story clearly details, he is "selfish" in that he does not like to share his garden, even when he is not around. Therefore, he decided to forbid people (literally people, and anything else in nature) to come near it.
As a result, the giant's garden begins to suffer the consequences of loneliness. Only hail, frost, snow, and wind can get away with staying right where they were, as nothing else could get inside.
Only in the garden of the Selfish Giant it was still Winter. The birds did not care to sing in it as there were no children, and the trees forgot to blossom. Once a beautiful flower put its head out from the grass, but when it saw the notice-board it was so sorry for the children that it slipped back into the ground again, and went off to sleep.
Arguably, the garden itself is at the center stage of the action and most of it affects it directly, however, the garden itself is not a character itself. Since the entire action falls upon the giant and affects him entirely, then he is the main protagonist.