That title would go to my old GT cichlid, Eleanor Rigby.
I had that GT in a pond and I wanted to find tankmates for it, so I went to my local food market and bought a live Mozambique Tilapia. The tilapia was about 2 times larger than the GT, so I thought it was fine. Not even 2 hours later, I had to evacuate the tilapia because my GT had already got to work bullying it. After that, I was very careful choosing tankmates, also the process I introduced them. One day, I decided on a jag cichlid that was about 4 inches larger. I made sure that the GT wouldn't try anything, yet it did. For about 3 days straight, he would non-stop bully the jag, but left no physical damages, hence why I didn't evacuate the jag. After about a week, everything was fine. Eleanor Rigby died of an unknown disease. At the time, I didn't know how to treat the disease, nor what it even was, because I didn't have the knowledge. The jag is still alive to this day.