Topgun in NJ has been around since 1988 History - TopGun Paintball in Jackson NJ
I have played there once a year for the last 10 years, and it is always a great field to play. Plenty of terrain, and good refs.
Maryland suffered a few major declines in paintball fields... Outdoor Adventure used to have a field in Bowie MD, very near my current house, and one in Rosedale MD, very near my previous house. Unfortunately, due to zoning, both have closed along with their retail store
The Bowie field would have been my nomination for the best and oldest MD field, but alas, the field is no more
I have been playing at Pasadena Paintball Park. P3 is a mostly wooded field, and a great place to play. Unfortunately they have only run the field portion of the business since 2010, so longevity does not compare to places like PAP or Robinhood.