The House of the Spirits
Mainly Esteban García
It's hard to pin all the blame on Esteban García – after all, the author works very hard to explain that the conflict in this novel stems from problems that are far bigger than any one person. But García is certainly pretty mean and nasty, and we'd definitely call him the story's primary antagonist.
Alternatively, we can look at Esteban Trueba as an antagonist. He's often his own worst enemy – after all, he's the one who spawns Esteban García in the first place.