I think there is some bad info in this thread.
Unfortunately, a lot of players say that a disc is stable when they mean overstable. Overstable means that it will resist turning right when thrown right handed backhand and will have a moderate to severe left fade in the low speed portion of the flight. Stable means that the disc wants to go straight - not right or left. Understable means that the disc does not resist turn in the high-speed portion of the flight, and usually has little or no low-speed fade. All of these things are relative to your power and your personal armspeed-spin ratio.
There are a few discs that are high-speed understable and low-speed overstable. This can lead to some long shots on S lines, but most discs like this end up getting pretty squirrelly. Some of my favorite discs break in to be high-speed stable and low speed stable (no turn, no fade). Teebirds, Rocs, Gazelles, Sabres, and I'm sure a few others fit these criteria.
Now, to the Rocs. There are three different molds currently used for Rocs. Rancho, Ontario, and re-release San Marino. These molds get their name from the engraving on the bottom of the disc that states where they were made, corresponding to Innova's manufacturing facilities through the years.
DX Ranchos start high-speed overstable and low speed overstable. As they wear, they lose the low speed overstability but keep the high speed stability for a long time. By the time they have the perfect wear, they have virtually no turn and no fade. Many players refer to Ranchos beaten to this stage as "money."
DX Ontarios start high-speed stable (not over or under) and moderately low-speed overstable. They lose the low-speed overstability very quickly in DX, and then lose the high-speed stability shortly thereafter, making them good for right-turning shots.
I've never owned a Sanny in any plastic, so I can't comment. I understand them to be between the Rancho and Ontario in practically every aspect.
KC Rocs are Ranchos run in a slightly different plastic. They are more durable, so they have the flight characteristics of the DX Ranchos for a longer time. They eventually break in to have about the same flightpaths, it just takes longer to get there.
I won't get into the Champ and Star versions, since I haven't thrown them all.
When life gives you lemons, stick them in the freezer for a few hours, then throw them back, 'cause they're like rocks, man.