Because there are over 3000 species of cichlids worldwide, for me this is an almost impossible question to answer, luckily it has been narrowed down to American cichlids, bringing it to maybe a little less than 1000. If you were to throw in Madagascans ( equally my favorites) I'd be in a real quandry,
But probably the favorite I have owned was Nandopsis haitiensus, and funny someone mentioned they were impossible to get, when I had hundreds of fry, I had a hard time giving them away.

One that I'd now love to own, but haven't yet, is Nandopsis ramsdeni, and since I haven't ever kept one, no photo.
Another I have owned that would rate right up there with haitiensus, would be Tomocichla tuba, but this was so long ago, my photo was with a flip phone.