By the time we arrived in Charleston, we'd done quite a few tours, but this was by far the best. We decided to go with Adventure Sightseeing, which also offers unlimited on/off privileges as long as their buses are running. We were even given a schedule of their different routes and how long the buses run on each route.
Our tour guide, although originally from Texas, was extremely informative. Our tour covered many sights including the downtown Historic District, City Market, the Hunley replica, historic homes, war stories, beautiful parks, the Citadel military school, and more. Be sure to check out their website noted above for more tour info and prices.