The Capitol: This is the most obvious answer. The Capitol, with its opulent lifestyle and cruel games, is the clear antagonist. They are responsible for the Hunger Games, the oppression of the districts, and the suffering of countless people. President Snow, the Capitol's leader, is a symbol of their tyranny.
The Gamemakers: These individuals design and manipulate the Hunger Games, turning them into a brutal spectacle for the Capitol's entertainment. They actively work to increase the drama and danger, making them a direct threat to the tributes.
The Other Tributes: While they are all forced into the Games, many tributes view the others as their enemies. The need to survive forces them into conflict, and alliances can quickly shift, making the tributes both potential allies and rivals.
The System: Beyond individuals, the entire system of oppression enforced by the Capitol is the real enemy. This includes the poverty, inequality, and the lack of freedom that the districts endure. Katniss's fight isn't just against individuals, but against the system itself.
The Hunger Games Themselves: The Games are not just a competition, they are a tool of control and fear used by the Capitol. The Games themselves are a symbol of oppression, and Katniss's defiance against them is a fight for survival and freedom.
Ultimately, The Hunger Games presents a nuanced portrayal of conflict. There is no single, clear-cut enemy. It's the complex interplay of these various forces that drives the narrative and creates the tension and drama of the story.