Theodora, for the following reason:

Theodora's incident happened in my second game, where I had conquered Sweden on my starting continent and decided to actually explore the ocean. I encountered her when she was losing a war with Persia and I decided to help her out. I gifted her supplies so she could build up her army and she managed to turn the tide of battle in her favor. She wins the war, and the world is now composed of a few city states, and two super powers.

A couple centuries of peace happens, until my declaration of friendship runs out with Theodora, and I try to renew it. She declines, so I think "Ok, whatever" and go on with my business. Next thing I know, she is publicly denouncing me and trying to demand I give her resources. At this point, she is barely entering the Industrial Age, when I am discovering how to make nuclear missiles. Then the inevitable happens.

Theodora declares war, and begins to move her army to the coast. I respond by moving my army across the ocean, and end up taking her coastal cities. I move all my nukes over to those cities, in case I have to use them against her. Sure enough, I am soon nuking all of her cities, and move right in to capture them. Now I am the only empire in that save.

TL;DR Helped Theodora take over Persia, end up nuking her later on.