Forte Inc's Agent newsreader is really the only one I've ever used. I don't use Usenet that much anymore since most of my topics of interest are pretty well covered in on-line forums (and Usenet has pretty much turned into porn spam central).
There are probably some other good freeware newsreaders out there and Agent works well for free, just with some of the more advanced functionality disabled.
Having used nn for a number of years, I struggled to adapt to FreeAgent, but without much success. In fact, I dislike all Windows USENET front ends so much, I've all but stopped reading news. It is also almost impossible to find a shell acount that offers access to news because of the administrative hassle.
If anyone knows of an nn clone for Windows, please post.