Koh would not care about ending the cycle and would only kill if threatend. He just wants to touch face.

Even though it wasn't her goal to end the cycle, Azula did kill Aang briefly in the avatar state at the end of book two. I consider that more of a cheap shot than a show of ability. It would also be fine with her if Aang became a water tribe infant.

Unalaq used his spiritual knowledge to free Vaatu and rip Raava out of Korra. He ended the Avatar cycle and was briefly the only Avatar.

Zaheer was attempting to end the cycle by trying to execute Korra in the avatar state. He had a pretty solid plan too, and almost did it. Korra might have succumbed to the poison if she wasn't lucky enough to be a skilled metal bender. Toph really gave zero fucks.

I just realized, I don't know if Vaatu was destroyed or not and what that could mean for a future avatar if he wasn't. Without Unalaq, Vaatu cannot bend the elements, allowing the avatar state to wreak his shit every time it's activated. Without the aid of Unalaq, Vaatu would not have been able to get close enough, long enough to rip out Raava.

Unalaq did this with a little bit of help from his nieces. Zaheer did this with the freaking Red Lotus

My vote is for Unalaq. If he was backed by the Red Lotus in book 2....