There is a vital, rich and active competition on this site to be the most truly nasty derailleur in history. In this crucial contest, the Roto Sagittario looks set to podium.
The Roto Sagittario is a clear copy of the Huret Eco. But it is horribly finished, is made without noticeable accuracy and uses a guage of steel sheet that is simply too thin to stand up to real life. But the kicker is the plastic outer parallelogram plate. This looks splendidly nasty and reliably breaks around the adjustment screws.
To make something this bad takes some real skill. Respect.
This is a version that has a rivetted-on hanger plate. It is the only example that I have ever seem that does NOT have at least one broken mount for an adjustment screw.