I would say that a Clown Fish is my favorite, the one with the orange body and the white stripes along with the black outline, those guys are so cute.
My mom has two, a male and a female.
Too many to choose from really. Some I really adore but can't have due to size or reef compatibility such as orange spotted filefish, longnosed hawk, pufferfish, and lionfish. Some are wonderful because of their colors such as anthias or tangs, while others aren't brightly colored and fairly common but win me over with their personality such as lawnmower blennies and dragon gobies.
I can't pick a favorite. My favorite fish that I would love to own is the Blue Girdled/Majestic Angelfish. I also really like Emporer Snappers. Neither will i consider owning until I am set in a place I will be living for years. I simply adore far too many fish.
God there are so many... But I really have gotten to Love my Lyretail Anthias.. He is so full of character and commanding in the tank.. Along with my Coral Beauty.. But my favorite has been and will be my clowns just because of the antics that they pull on a daily basis.