Re: Star Trek III - The [Speculation Begins] (Spoilers)
The Whale Probe in STIV was certainly an antagonist, albeit an unintentional and unconventional one, but arguably the bigger antagonist of the film was 20th century humans. They're the ones who hunted the Humpback Whales to extinction, thus prompting the Probe to remain in Earth orbit awaiting a response, and they're the ones who most consistently represented an obstacle or threat to our heroes completing their mission throughout the film.
'First Contact' is the tale of a man who just wants to cash in on his creation so he can get wasted on an island full of naked women, but his fans keep insisting that he's a saintly visionary who has profoundly altered the world. AKA - 'I Don't Want to be a Statue: The Gene Roddenberry Story.'