For what's worth, I believe this is a fairly comprehensive list --
Six Flags New England -- formerly Riverside
Six Flags Great Adventure -- formerly Great Adventure
Six Flags La Ronde -- formerly La Ronde which was formerly Expo '86, I think.
The Great Escape
Six Flags America -- formerly Adventureworld
Six Flags over Georgia
Six Flags Kentucky Kingdom -- formerly Kentucky Kingdom
Wyandot Lake
Old Indiana Fun Park -- Six Flags' boneyard, no longer an operating park.
Six Flags New Orleans -- formerly Jazzland
Six Flags St. Louis -- formerly Six Flags over Mid-America
Six Flags Great America -- formerly Marriott's Great America
Six Flags Astroworld -- formerly Astroworld
Six Flags over Texas
Six Flags Fiesta Texas -- formerly Fiesta Texas
Frontier City
Six Flags Elitch Gardens -- formerly Elitch Gardens, new park with the same name as the original, though in a different location.
Six Flags Magic Mountain -- formerly Magic Mountain
Six Flags Marine World -- formerly Marine World Africa USA
Six Flags Mexico -- formerly Reino Aventura
Wild Waves and Enchanted Village
...and this ignores their waterparks entirely. I list these only because the Six Flags site doesn't list the parks' former names.
All of that being said, I like some Six Flags parks more than others and rarely have had a poor time at one of their parks. Though, it's pretty hard to upset me when I'm on vacation because I'm there explicitly to avoid stress and so I tend to roll with the punches a bit and take things as they are.
I've visited seven Six Flags parks, one of which was Six Flags Worlds of Adventure before the park's recent sale to Cedar Fair. Six Flags Magic Mountain was, hands down, the worst park I've visited simply because of its horrible operations. I still haven't decided if I'll ever go back when on vacation to Southern California.
Six Flags over Georgia, however, is a gorgeous park steeped in history and I had a splendid time there. I wouldn't hesitate to stop there again if given the chance.
Above all else, these parks are amusement parks, albeit mismanaged, and are not trying nor meant to compete with Disney theme parks. Walt Disney World is a destination -- something people entire vacations around. Six Flags parks are, as one of their advertising campaigns once pointed out, for day trips. They're a diversion that allow you the chance to do some neat things you won't see elsewhere -- say, fly like Superman -- but they're not purporting to be an encompassing, transforming experience. For that, save your pennies and go somewhere like, oh, Disney?