Whether or not you're fully on board with Samson's vehement condemnation of his ex-wife, it's obvious that for Samson, she's enemy number one. To be fair, she did betray his secret and land him blind and in prison, so this is definitely behavior that qualifies you as an "antagonist."
But we also think Dalila is Samson's antagonist because she's the only character in the whole poem who actually stands up to him. Harapha might be big and mean, but he just makes a bunch of empty threats. Dalila, on the other hand, actually challenges Samson's assumptions about his actions and her own.