Zhao's actions were inarguably worse. Not only did he almost wipe out all waterbending on the planet, he attempted to wipe out the last remaining stronghold of the Water Tribe. Gilak was just as racist, but he also had a legitimate concern about the Southern Tribe being made a colony of the North, which it was by Korra's time.

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?