Your neighbor. It is accepted law in all states that a tree whose trunk stands wholly on the land of one person belongs to that person.
If the trunk stands partly on the land of two or more people, it is called a boundary tree, and in most cases it belongs to all the property owners. All the owners are responsible for caring for the tree, and one co-owner may not remove a healthy tree without the other owners' permission. As for who owns any fruit from the tree, however, that's another matter. Check out the details in Neighbor Law: Fences, Tress, Boundaries & Noise, by attorney Cora Jordan and attorney Emilt Doskow (Nolo).