The Simplex 410 and 610 gets my vote as about the worst derailleurs ever made. Or least among the dozens of different derailleurs I've used and worn out, these were the worst.

Here is the problem: the parallelograms are made of plastic, covered by a thin cosmetic metal cover. The steel pins that hold the parallelogram together rapidly wear out the holes drilled in the plastic.

Peugeot Prestige derailleur (SX410)

To recap: the parallelogram plates look metal, but they aren't. I wore one of these out in a single summer. Even when new, these were not the 'tightest' derailleurs made.

You want the all-metal 630 or 6600 units. I was riding my Gitane with a SX630 over this week, and it is a great piece of kit.

Indexes beautifully with a Suntour Winner Pro 7-speed freewheel, and Campagnolo 9-speed Ergopower shifters. But this is just a happy accident.