The NPC is by far the largest but they have a long way to go regarding natural (tested) events. Even "natural" shows in the NPC may be random at best.
The OCB has gained a lot of ground in the last few years, however the INBF and others are great for natural athletes as well.
The link below leads to a page listing many natural organizations that sanction natural bodybuilding competitions
For Natural, compete in an exclusive Natural Organization such as the NANBF, OCB, or INBF, definetley stay away from the NPC. They do have a few Natural shows, but your $ spend and attention will be fed to the non-Drug testing headquarters of the NPC.
Natural Bodybuilding has grown in popularity and is now LARGER than the non-Drug tested world of the NPC. As for as organizations, the top Natural Organizations are:
NANBF (IFPA Pro)
OCB (IFPA Pro)
INBF (WNBF Pro)
This is the way to go. I've competed in competitions in both MN and FL, and these organizations have consistent, sound drug-testing techniques to ensure 7 years of drug-free. Also, the professional ranks of each are excellent with cash prizes for top finishers.