OK, so... let's get something clarified.

There is a difference between an interesting antagonist and an entertaining one.

Antagonists who don't have very deep characterizations, or ask you to think too much, but still hold your attention are less interesting and more entertaining. Antagonists like OP listed, who hold your attention through their witty banter, excellent showmanship, and overwhelming charisma. Even if you hate their guts, they just have so much fun being evil you can't help but smile with them.

The problem with Zenos and Fandaniel (at the moment)? They're neither interesting nor entertaining.

As I see it, Zenos is like some kind of Uchiha Madara figure - famed in-universe as an unbeatable warrior, swats aside everyone who goes up against him like a fly. Whenever his back's (seemingly) up against the wall, he pulls a new superpower out his ass. Trained so you don't get curbstomped again like in Rhalgr's Reach? Well, out comes Ame-no-Habakiri to ruin your day! Trained some more to match up to him even with that? Well, he has an artificial Echo and is gonna use it to possess Shinryu! Killed himself? His Resonance lets him live on! Stuck in a crappy body? Beat the crap outta the guy possessing his original (superpowered) one and take it back! Stuck in the throne room after assassinating his father, facing off against two of the finest warriors in the known world? Swat them like flies, then teleport away Ascian-style! Weird guy comes chatting at you? Well, now he can see soul colors!

He just gets more and more and more powers, often inexplicably. And unlike Madara who eventually started having fun, Zenos just sits around with absolute apathy for everything going on around him. He does not give a flying flock about anything, anything at all, except for his desired rematch. He's not interesting anymore, and he was never entertaining.

Fandaniel's problem is that he doesn't seem to have a motive at all for doing what he does. I've seen a lot of people liken him to Kefka and Kuja, and while the former has some merit I just don't see the latter. Kefka actually did have a reason for doing the things he did - he wanted to prove the meaninglessness and worthlessness of life, and it's implied in Dissidia only found fulfillment in life through meaningless destruction. Kuja, on the other hand, did everything he could to avoid death - everything he does up until the end of Disc 3 is an effort to prove himself superior to Zidane and thus (more) deserving of life. It's only after he learns his days are numbered that he goes on the suicidal cosmic temper tantrum, and even then he does provide (obnoxiously arrogant and selfish) reasoning behind his actions.

Fandaniel... has no such reasons. He just says "I'm going to destroy the world and then kill myself because... I can! Don't try to reason with me, I have no reasons!" and flies away. He provides no reason whatsoever, nor does he explain why he intends to kill himself only after killing everyone else. If he wants to die he can just do so and leave everyone else out of it.

There may be more to Fandaniel than that (Ryne tries to appeal to Mitron's humanity in Eden; I get the feeling that may be a part of his motive but that's all I got) but as it stands his goal is just to kill for the sake of killing. It's neither interesting nor entertaining.