I’d say Ozai considering he wanted to burn down the Earth Kingdom.

While ozai did crave power and was cool with doing whatever it took to gain it. he had at least one redeeming quality which was his admiration (i hesitate to say love because people would crucify me) of azula. and even after he was put in prison showed some sort of mild respect for zuko.

Unalaq purely wanted power. he wanted to be the most powerful person in any capacity and was cool with whatever came with that status. I don't think unalaq truly wanted the world to be dark and chaotic but was fine with it as long as he could become the avatar. unalaq truly showed no sort of redeeming qualities and was fully manipulative from the beginning. he will, in a heartbeat, attack his niece simply for disagreeing with him. he abandoned his comrades and covered his association. yeah unalaq is pure scum

I say Unalaq mainly on the reason he tried to end the world itself.

I would say Unalaq is the more evil of the two for one simple reason: Ozai was raised in a culture that encouraged his cruel, domineering behavior and exalted his prodigious power. His genocidal plans are little more than an evolution of that ideology.

Unalaq grew up during an era of peace and stability, with a healthy culture that discouraged senseless violence and greed. He had everything he could possibly need handed to him by birthright, yet he was so power-hungry and obsessed with the spirits that he rejected his own humanity and tried to destroy the world.

Unalaq killed sokka lol

And waned to end the world and become a dark avatar

And so he did but not end the world

Unalaq did not killed Sokka as his fate has never stated.

@G4ORCE Who told you that Unalaq killed Sokka?

"These are just guesses"