I'd probably say Demon at Six Flags Great America is the roughest one, however, it's still a really fun ride. If they would just reprofile the the drop off the midcourse/entrance to corkscrews, it would be fun. That's the only rough spot, but it's very bad.
Suprisingly, Shockwave was very smooth for my ride. It was also very intense, and very smooth. It was as smooth as Kraken at Sea World (I know, unbelievable.) But, I'm telling you, if you sit in the back car of the Arrow multi-loopers, it's smooth. (I think the trailer is back there, which makes it smoother.) I guess that technique also works on Ninja at SFOG. Can anyone confirm that?
In comparison, I've been on:
Shockwave, Six Flags Great America
Demon, Six Flags Great America
Cedar Creek Mine Ride, Cedar Point
Gemini, Cedar Point
Corkscrew, Cedar Point
Magnum XL-200, Cedar Point
Python, Busch Gardens Tampa