BY FAR!

I feel like the Dark Swan arc had a lot of wasted potential, in that she never really embraced or at least gave in to the darkness, which I think would have been amazing to see, and to not go there left me disappointed. But Killian was just annoying & weak as a Dark One, imo. Like, I don't blame him (he literally begged Emma to let him die rather than become a Dark One), but I really didn't like him.

^ Agreed. With literally all of this.

I feel like Killian achieved a lot more as Dark One than Emma did. He came closer than anyone to finally shattering the Dark One's link to the Dagger and give the DO complete power, and only didn't achieve it 'cause he had a change of heart. I had my problems with the Killian-as-Dark-One twist, but objectively, he was much better at it than Emma was (probably 'cause he had a lot more darkness in his heart to begin with.)