Who is the epic hero in The Iliad? Hector or Achilles?
You are, I imagine, referring particularly to Book 22, which narrates the titanic final battle between Hector and Achilles and then Hector's death. As to who is the more "heroic", there are arguments to show that both are not heroic at all. For example, Hector fails to uphold honour when he flees from Achilles, but arguably he regains his honour when he decides to fight:
"So now I meet my doom. Well let me die-
but not without struggle not without glory, no,
in some great clash of arms that even men to come
will hear of down the years!"
Clearly Hector displays great nobility in accepting his death but also with his desire to not bow out easily.
On the other hand, Achilles most clearly shows eager determination to avenge the death of Patroclus, which is a key requirement to be a hero. But some critics in what he does with Hector's body he actually goes to far and loses the right to be known as an "epic hero":
So he triumphed
and now he was bent on outrage, on shaming noble Hector.
Piercing the tendons, ankle to heel behind both feet,
he knotted straps of rawhide through them both,
lashed them to his chariot, left the head to drag
and mounting the car, hoising the famous arms aboard,
he whipped his team to a run and breakneck on they flew,
holding nothing back.
Notice how the text calls this act a "defilement" of Hector's body, emphasising how profoundly unheroic this act was.
This to me indicates that Hector is the more "heroic" in this tale, as, although he did display cowardice, at least met his end honourably and in a heroic fashion, whereas whatever kudos Achilles gained through his victory he lost with how he treated Hector's body.