My favorite entry on this list: The Ball.
Ozma is strange, and I think legitimately frightening when you give it some thought. Really think about Ozma. A completely optional fight that you only encounter if you trade in years off your life to complete the Chocobo Hot and Cold treasure hunt side quest, which is the longest side quest in Final Fantasy IX, which is already full of long side quests because it's a Final Fantasy game. Once you've successfully cashed in your human energy for a digital reward in an old, old PlayStation game, you are granted admission to a cave tucked away in the back of the Air Garden. And there's Ozma.
Ozma doesn't talk. Ozma doesn't heal. Ozma does nothing but try to end your life. Just non-stop, never-ceasing dull thuds coming straight at you and your party the moment you're in the same zip code like nuclear god damn bombs. And Ozma is aiming for the throat. Ozma casts Curse? You're dead. Meteor? Dead. Have a party member susceptible to Lv5 Death? Not anymore because they're dead. Or it'll just cast Death and skip the formalities altogether. Oh, and Ozma has counter attacks, so watch out for Berserk and Curaga because they're in the mix once it hits a certain HP threshold. This thing has no brakes and no interest in explaining itself to you or anyone else, so your victory must be earned.
What I find especially cool about Ozma is how little we know about it. Just to reiterate, this is not the final boss or a mandatory boss or anything. Ozma is a completely 100% optional fight. Mene warns you when you approach the cave and says he "feels a strange presence...not of this world," but that's it. Eidolons are creatures that come from people telling legends, similar to personas, but Ozma is just a colored sphere. Maybe it's a legend so old that everyone who exists has simply forgotten what it is, and it's attacking you because you came into its home as the first visitor in eons, Shadow of the Colossus style.
Creepy, beautiful, mysterious, and a giant pain in the ass, Ozma certainly makes its ancestor Warmech proud.