I'd have to say my black ghost kinfefish for distinctiveness. My Iirian red rainbowfish for his golden red color. And also my severum for his impressive size and disappearing and reappearing stipes act...
Then I'd have sto say my shocking pink male anthias (in saltwater) and also my wrasse for her striped yellow and pink and blue face mask that glows neon under flourescent lights.
D'oh! Kaskato is right... They're all unique.
They have intricate patterns that can differ greatly between individuals, but retain all the natural beauty of fish that haven't been "tinkered" with for color or finnage. Plus, they can change this pattern at will, allowing them to take on a different look at any given time.