I think people should also post why their favorite fish is their favorite fish. I'm sure everyone has different criteria which makes a certain fish their favorite.
My favorite is the Chocolate cichlid. By far the most personable fish I have come across with the exception of a Oscar. Basically the same personality as a Oscar, but I prefer the Chocolate cichlids colors and love the fact that Chocolate cichlids change their colors with their mood. It's very easy to gauge if I am stressing out my fish or he is excited and displaying.
A few pics of my buddy Louie (still growing)-
Day of purchase at 3"
Nowadays at 9"