Of the ones I've been to (missing Over Georgia, Mexico and Frontier City so far), I'd put them into the following categories - based on my experiences and days at those parks.
Bottom-Tier
Six Flags America - dump.
Six Flags Darien Lake - see above, but at least I got the Superman cred at this one.
Six Flags St. Louis - the park itself isn't bad, but it's not exactly great either.
Six Flags Discovery Kingdom - I went pre-RMC, and didn't think the place was up to much.
Great Escape - pretty, but the coaster selection is crap.
[I'd hazard a guess that Frontier City would slot in here somewhere]
Mid-Tier
La Ronde - bottom of the mid-tier, but I really enjoyed my day here and I quite liked the inner-city-but-not-really vibe it had going on.
Six Flags New England - I went pre-RMC, but still thought this had a pretty decent coaster selection.
Six Flags Fiesta Texas - a decent park marred by a rather heavy handed bad weather policy.
Six Flags Great America - top of the mid-tier, but I also only went pre-RMC (and Maxx Force) so it might jump up a tier if I were to revisit.
[I'd hazard a guess that Mexico would slot in here somewhere]
Top-Tier
Six Flags Over Texas - actually had a really great day at this park.
Six Flags Great Adventure - a pretty solid line up, and home to a few of the true icons (albeit none of them are 'stunning').
Six Flags Magic Mountain - best of the lot, in my opinion.
[I'd hazard a guess that Over Georgia would slot in here somewhere]