In a story full of heroic characters it can be hard to identify the central hero. The other educator answer here makes a good case for Hector. He does seem to be the most heroic hero in the epic poem, truly embodying the aspects of a hero by modern definitions. However, one might also make the case that Achilles is the central hero of The Iliad.
Achilles' centrality is established right in the opening lines of The Iliad. The poet opens by invoking the muses to
Sing, O goddess, the anger of Achilles, son of Peleus, that brought countless ills upon the Achaeans.
Right from the start Achilles is identified as the main character which the epic will follow. Specifically, The Iliad is the story of Achilles' anger. It begins with the cause of this anger and follows the consequences of it. The epic ends with the resolution of Achilles' anger when he returns Hector's body to the Trojans. Looking at The Iliad this way, it is clear that Achilles is the central hero of the story.
Achilles does not always come across as the most sympathetic of heroes. He is moody, self-centered, and self-serving. However, he fits the classic definition of a hero according to the Greeks. He is a warrior from the legendary past with near-superhuman abilities, who is descended from the gods. Achilles fits all these requirements.