The chief antagonist in The Hunger Games is the totalitarian regime of the government of Panem. From the very beginning of the novel, the narrator Katniss familiarizes the reader with the many cruel and oppressive laws and rules of the ruling government, which has resulted in deprivation and poverty for the citizens of District 12. The vicious rule of the Capitol has enacted the Hunger Games, the primary focus of the novel, as retribution for a past failed rebellion.
In addition to the primary antagonist, the Capitol, Collins builds suspense and drama in the arena of the Hunger Games by pitting Katniss and Peeta against some chillingly lethal adversaries from the other districts, like Cato, Clove, and Glimmer.
I would say President Snow because he started the Hunger Games.
If you just consider an antagonist to be an opponent, then there are plenty more antagonists' than President Snow.
Cato: One of the careers that participates in the Hunger Games; Tries to kill Katniss multiple times.
Glimmer: Another one of the careers that participates, killed by Katniss.
Clove: Another career, almost succeeds in killing Katniss, but is stopped by Thresh.
Almost anyone participating in the hunger games could also be called an antagonist, because of their competition against Katniss, and attempts to kill her.
Well, President Snow.
Then everyone against Katniss in the Hunger games.
Right. So basically, what everyone else has already said. :)
President Snow is only one of many antagonists but he may not be the true one. He is in power as an elected leader (president). Therefore maybe his predecessor(s) may have made the hunger game.
In my opinion I believe that the antagonists are the citizens of the Capitol. Every one there celebrates the games as a festival. They try to make a utopia by teaching the other districts who is in charge by sending teens to fight. They make other districts hand over tributes to die and they even included an escort so they wont run away.
In the games its self the antagonists are mainly the fighters from district 2 (Cato and Clove). Especially Cato; when he saw the score that Katniss got he knew that she was his number one target.
I agree, President Snow.