My favorites, as in effective, and pretty safe with predatory fish, are raphael catfish. Some of them, like the striped and spotted ones, look pretty attractive, have a never-ending appetite and a fairly large mouth to swallow big chunks. And since they are nocturnal, they are really safe with like piranha's, since they are only active when the p's are sleeping.
As far a appearance, my (shared) number one are all those fancy pleco's with great coloration and patterns. Unfortunately, they often are not very hardy, really expensive and not suitable for a tank with predatory fish.