The original Six Flags park is Six Flags Over Texas in Arlington. The park was opened in 1961 as America's first ever regional theme park. There were 6 themed areas to match the six flags. The six flags represent the 6 nations whose flags have flown over Texas. Mexico, Spain, France, Texas, the Confederacy, and the United States.
There were eventually 2 other Six Flags parks that formed the original 3 Six Flags parks. Six Flags Over Georgian and Six Flags Over Mid-America (now known as Six Flags St Louis). I'm not sure of all the details, but I think it was under the control of Warner Bros. that Six Flags acquired Great Adventure, Great America, Magic Mountain, and Astroworld. Once Premier took control, all heck broke loose and now we have all of the Six Flags parks we have today, sadly, at a bit of a loss of quality at many of them.
I still love going to the original SFoT, which just happens to be my home park. It's a wonderful park, and, from what I hear, is still one of the better of the Six Flags parks.