My view of the 'perfect' main antagonist and the 'ideal' main antagonist is vastly different from one another. The perfect one would be the basic archetype; powerful, uses others as minions, wishes to rule/destroy the world or something similarly grand, and cannot face defeat except for at the hands of the main character. His convictions hold fast, he is arrogant, selfish, and views himself superior to all. His strength and brains are flawless, except for the one deviation that causes his downfall; the main protagonist.

However, I do not need ANY of that to find the ideal antagonist. No complicated, large scale plan is necessary. No strength or ambition necessary. He can even be weak enough to be beaten by an angry little girl.

All he needs to be is the main protagonist's enemy. In fact, from my point of view, all characters are just that; several different individuals with different beliefs and personalities. They may not even know each other, but due to a difference in goals, they may but heads and try to kill each other. But once its all over and done with, they continue their lives and may occasional run into each other, argue, fight, or cooperate depending on the situation and their personality. Sure, it may sound odd, but after going through the To aru Kagaku no Railgun manga and To aru Majutsu no Index light novel, I really see that as the ideal situation. It's realistic and interesting, and the point of who is good and who is evil is completely pointless. The same goes for Fate/Zero, where every master and servant was an individual with goals, and came together or fought depending on their moods, interests, and goals. Who was the main antagonist? They all were! And, at the same time, there was none!

Tough question. I think there's a lot of different ways an antagonist could be optimized along any of these lines. For example, Aizen is probably one with undeniably evil motivations and acts as the strongest character for a time, and I felt, at least for a great deal of the time, that he was one of the best antagonists I've seen. There are plenty of good antagonists in anime that fit a number of these characteristics. And I agree with your assessment, exorcist, that the role of antagonist need not be so straightforward. I like a situation where there is no obvious antagonist, where even the protagonist has questionable motives and there is no black and white.

But, if I was looking for an ideal one, I have to go with what made me love Kyuubey from Mahou Shoujo Madoka Magika so much. It wasn't his strength - in fact, his real strength wasn't even revealed until very late in the series - but rather the way he approached the world. I don't mean to spoil this for anyone, but his entire goal is to keep the universe from falling apart by providing it with a constant and ready source of energy. And some of his methods to get there actually seem quite noble. He grants little girls their wish and gives them powers the likes of which they could never begin to imagine to fight evil witches that are killing people in their cities. That all ends up being a showy gloss to cover for the fact that he removes their souls and places them in separate vessels, essentially making them into empty dolls, and that he's actually rooting for those very souls to become corrupted by encounters with the witches to create yet more witches, which are the source of that very power he needs. His complete dispassion for human feelings is perhaps the most ominous, especially coming from a creature who looks adorable. It's one of the most disquieting things I've seen in anime.

So I suppose it's more than my answer that makes him interesting, but the answer itself is the core of what I find so fascinating. I suppose I could find an antagonist that outdoes Kyuubey, at which point I'll change my mind, but for now this is the kind of antagonist I like most.

All of these have the potential to be good characters, it simply depends on whether the character is well-developed and is likable in a sense. I agree with Whiteflame about Aizen, simply because he was a good antagonist and he probably would be classed as more than one of the definitions above... For example, after he seceded from the Soul Society, he was clearly a strong antagonist with evil intentions, however, during the bit before he does and the Time Skip Arc, he was a villain with unclear motives, who had a sort of secret identity, since he was acting as a Lieutenant, even though he was evil.

The tragic villains are always the best. They have the most depth and character too them. The ones that are evil, but they make you feel sorry for them and get you crying if they die.

^ Yeah, I agree. I guess it helps you to understand their story, too. Well, at least, I like villains like that, but I think all of these definitions can create a good antagonist.

An antagonist who has good intentions but carries them out in a terrible way is quite interesting to me.
Also one who has a tragic background. It makes the character ever the so more interesting and makes you love the antagonist.

My idea of the ideal antagonist or protagonist changes with the anime I am currently obsessed with.

^ XDDD

I do think antagonists like these are the best. They get you thinking about their ideology and I would sometimes agree with them, but the protagonist would always find a way to prove him wrong.

Although ones with the tragic past have a deeper character and depth.