Quality is judged on a number of factor. Several would include the size and number
of instruments (or in the case of teleseismic and regional networks the number of
reporters); the past quality of locations and epicenters judged on comparison with
ISC or NEIS. For example, NOA has maintained a local catalog for nearly a century.
The data is consistent (for larger events) with that determined by ISC after ISC gets
teleseismic data from hundreds of sources. On the other hand, NORSAR (in Sweden)
often has events listed which are hundreds of km distant from the epicenters given
in other catalogs.
This is a very difficult thing to define. In general I consider the highest quality
teleseismic catalog to be ISC followed by PDE and the Russian catalog (GSR).
Local networks generally do a pretty good job, but those which have been in operation
the longest generally have the best quality data (mostly due to trial and error and
money) - these would include SCSN, NCSN, UUSN, IGNS (New Zealand), KAN (Turkey),
and various networks in Europe.
The question of quality is often best answered by people who are retired or near
retirement and who have had a lifetime of experience to answer it.