Who is the main antagonist of The Avengers?

It's a bit tricky to pin down a single main antagonist for The Avengers, as the movie features a complex web of villains.

However, the most prominent and central antagonist is Loki, Thor's adoptive brother and the primary driver of the film's conflict. He orchestrates the invasion of Earth, seeking to conquer and rule the planet.

While other antagonists like the Chitauri (an alien army) and the agents of HYDRA play significant roles in the film, Loki remains the driving force and the primary antagonist who directly confronts the Avengers.