It seems pretty clear to us that Troy Maxson is the protagonist of Fences. He's the center of every scene. Even in the last scene of the play, after he's dead, all anybody can do is talk about him.
Troy totally drives the action of the play; his deeds and misdeeds keep the drama going. It's Troy who puts his brother away in a mental hospital. It's Troy who has an affair. And it's Troy who crushes his son's dreams of college football and kicks him out of the house.
Notice that a lot of these actions aren't particularly nice. Though Troy definitely has good qualities, a lot of what he does is bad, bad, bad. He's a great example of how the protagonist doesn't always have to be a "good guy." With Troy Maxson, August Wilson created a deeply flawed and deeply intriguing main character.