When it comes to as a person that would likely be zhao. He literally killed the moon out of pride and ambition.

However as an antagonist zhao was excellent. in fact I would almost put him on par with azula (yes I would) when it comes to antagonist criteria. While he is undercut a bit in the second half, he presents a great threat for the gaang and the narrative, he has a lot of power, the will and desire to use it and he is competent. Also he presents a threat to both sides of our main characters including iroh and zuko

What did Gilak do?

What do you mean worse? Because Zhao was a very good antagonist. Or do you mean which character acted the most evil?

Scothe, that's a good point.

RedCoat& ColourfulBag, he attempted to start a revolution, attempted to kill the leaders of the Earth Kingdom and Fire Nation, had people abducted, and attempted to commit murder several times.

Sounds like a lot of attempting not a whole lot of getting things done.

Scothe, Gilak wasn't the most intelligent antagonist. Not by a longshot. He's arguably one of the least-intelligent overall. Granted, his shortsightedness isn't exactly an uncommon trait among antagonists in this franchise. In fact, his strategic myopia is a trait shared by the overwhelming majority of the antagonists in Avatar.

As short sighted some other antagonists might by they still accomplished things, albeit not thier overall goal as those pesky kids always got in the way, but did atleast made strides.

Zhao. He was a crazy idiot. Gilak had somewhat of a point.

Villains with plans are easy mode, now insane villains those are the ones you gotta watch out for.