Antagonist

The Suitors (especially Antinoös and Eurymachos)

The suitors are in the process of eating poor Penelope and Telemachos out of house and home. They transgress all traditional ideas of honor by proposing marriage to a woman still waiting and grieving for her husband. They don't show any respect to the man of the house, Telemachos. Oh, and they also beat up beggars.

Poseidon

Poseidon is a more distant villain than the suitors, working his magic from afar and never confronting Odysseus face-to-face for any sort of showdown. Of course, this makes it difficult for Odysseus to best or beat Poseidon in any sort of traditional way; instead, he has to make peace with his enemy after suffering the hardship of his punishment.