Balloon bellied mollies are repulsive, as are horrendously modified goldfish, like ryunkins. (Some of the "modified" goldfish aren't as bad as some others, but in general, all but comets are just sad to look at.) Parrot cichlids are also deformed-looking . . . I don't mind most of the cichlids; they're born with an agressive nature due to their wild habitats and really can't help that. Some have really exotic, almost flourescent coloring, so I think (can't talk out of experience yet) that a tank with them would be lively.
Interesting, how most of the posted replies refer to "modified" fish, and few deal with natural wild ones.