Depends on what you wish to see and how long you plan on staying.
James Island is by far the nicest and most often recommended. It has space between the sites, nice walking paths including dock to tidal creek, 5 minutes to downtown by toad, and easy access to Folly Beach. Going out of town for sight seeing is usually pretty easy once you get off the island since you then start going against rush hour traffic.
Oak Plantation is nice and great base for seeing the islands, plantations on Hwy 61, and a trip or two into town. Once rush hour clears by about 9:30am, it's a relatively quick 15 minute ride into downtown. Afternoons you either need to be back to the campground by 3:30 or just hang around downtown until about 6 unless you want to take over an hour to get back to the campground. Traffic isn't nearly the problem if visiting the islands, south of town, or plantations.
Equal to the Oaks is Mt Pleasant KOA. Once was way out of town, but now almost next door to Walmart and strip mall, but not to close. Since it is north of town, easy to get to Georgetown, Boone Hall Plantation, Bulls Island Ferry, and Patirots Point. Traffic is about the same as Oaks.
