Nearly all the commercially available usenet service providers are running off the same 5 mega newsgroup server databases

Giganews
Highwinds
Readnews
XSNews
Searchtech

We build and maintain a massive table of all the providers and which back end servers they are actually drawing from.

By our counts, about 80% of the services are actually run through just the top 3 mega newsgroup servers.  Our recommendations are strongly biased toward providers based on the Giganews, Highwinds and Readnews back ends because we know they have been reliable and follow all the legal guidelines for content, etc.