I think Unalaq and Vaatu prevented Ozai to be the franchise's sole main antagonist.

The main reason why Ozai and Vaatu can be considered as the franchise's two main antagonists because Ozai acted like the franchise's sole big bad before Unlaq and Vaatu prevented him from this.

Vattu is known to be a background antagonist in the Avatar books and the franchise's greatest threat.

Vaatu is a threat that is 99,(99)% more likely to be even close to it in the avatar. but as an antagonist, he is not the main one.

Honestly, Vaatu didn’t feel like an antagonist. I mean it is harder to treat him as anything but a joke when he is a giant flying spirit magic carpet with special spirit powers that don’t make that much sense. Ozai was cool and his firebending skills made him much more serious and scary while Vaatu didn’t