I think you could safely state that the Bethlehem Loading Company (Belco) ruins are the most extensive standing ruins anywhere in the Pine Barrens. This is almost my back yard, and I have explored pretty extensively. There are quite a lot of ruins to be seen from the trails, but there are much more deep in the woods, in places that are almost inaccessible due to the heavy, thorny thickets and swamps. Many of them are in remarkably good condition, and in discussing this with some senior members on the site, I decided not to reveal any details.
A good place to start is the park website:
http://www.aclink.org/PARKS/mainpages/estell.asp
Also here:
http://www.aclink.org/PARKS/mainpages/historic.asp
Estell Manor Park and Belco are the same thing. Weymouth furnace is interesting and worth stopping by if you're in the area, but there isn't a whole lot to see.