lol, nice one Ziggy!
I'd say that mine is a toss up between mollies and black skirt tetras. The mollies don't understand concepts like "abstinence" (I've since gone to all females, about 5-6 months ago, thank god the fish are finally done popping out fry), and they do poop a lot. Black Skirts because I've had no luck with them since I lost my first stock due to something that got in the water during work on the house.
However, convicts (of which I have only ever had 1) would definitely be up there. I don't want to get any every again!