Who are the antagonists in The Hunger Games?

The protagonist is Katniss Everdeen, so the antagonists are those characters who oppose her. The main antagonist would be President Snow. He is the leader of the corrupt dystopian government that oppresses its people and pits them against one another. The government is responsible for the poor conditions in Katniss's home district and for the Hunger Games competition itself.
While in training and in the arena, Katniss gains some enemies that may be seen as secondary antagonists. Cato, along with his band of followers from the wealthy districts, are Katniss's primary opponents during the Games.
Katniss reinforces the idea that the government, represented by President Snow, is her main antagonist when she and Peeta decide to eat the berries that will kill them both at the end of the Games. The competition is the manifestation of the tight hold this repressive government has on the society. Katniss refuses to play by the rules, thereby opening the door for the revolution that stirs in the sequels. Consider the following passage:

There’s just one crown, though, and you can hear the crowd’s confusion...until President Snow gives it a twist and it separates into two halves...He’s still smiling when he settles the second on my head, but his eyes, just inches from mine, are as unforgiving as a snake’s. That’s when I know that even though both of us would have eaten the berries, I am to blame for having the idea. I’m the instigator. I’m the one to be punished.

Though Katniss's actions have been subversive throughout the novel as she resists the governments authority, it is this choice that most directly challenges the system. By refusing to kill Peeta, Katniss shows the nation that the government is the true evil.