Trigon could be the show's main antagonist because he coldly betrayed he's pawn Slade in Season 4, he's the show's greatest threat and the show's most evil. If Trigion is the show's main antagonist that might make Slade the show's central antagonist.

I spelled Trigon's name wrong by accident.

Simply being the greatest threat doesn’t mean they’re the main antagonist. In fact, that actually helps them be the overarching antagonist, I.e. Unicron is only even considered OA because he’s more dangerous than Megatron. Besides, Slade is the Teen Titans’ most personal enemy

Um.. no. Slade is the main antagonist in two seasons, the secondary/central antagonist of one season, and also appears in other two seasons. He is the most personal enemy to the Titans and the most dangerous villain in the show (dangerous, not powerful or destructive). Overall he was a very consistent threat.. you know, when he wasn't dead. Trigon, on the other hand, is active during one season and one episode. Yes, he's more powerful than Slade. Yes, he killed more people. Yes, he had bigger plans. But he's simply not present in the majority of the show, unlike Slade.

Central antagonists of a TV series and film too can have a can have a personal connection with the main protagonist than the main antagonist does, these central antagonists are Tomura Shigaraki, Darth Vader, Kylo Ren from The Force Awakens, Jack Torrance from The Shining, Clarence Boddicker, Pat Rollins, Phineas T. Ratchet, Gunmar, Zigzag, Chris D'Amico, Humpty Dumpty from Puss in Boots (maybe because he was mostly Jack & Jill's bosses), Kaecilius and Tempest Shadow.

I could've showed you more central antagonists it's just I showed enough.