Pyke, flints finger and seagard 50/50 - 100%
Riverrun 48/50 - 96%
Moat cailin 41/50 - 82%
Lannisport 28/50 - 56%
Harrenhal and winterfel - 22/50 - 44%
White harbour - 20/50 - 40%
Highgarden - 4/50 - 8%
Crackclaw point & starfall - 3/50 - 6%
The reach and oldtown - 2/50 - 4%
So some insights that might not shock veteran players but will help steer newer players in the right direction:
Lannister is apparently easier to conquer than stark despite the current meta (what this data doesn't tell us is how many times stark wins if you go after lannister). The rest of the group 'siding' with lannister in gj/lannister war doesn't seem to help much.
You will not win if you cannot hold seagard or flints finger. To this end you should park a ship in bay of ice and outbid stark on the iron throne for raids. It's much better for Stark to keep the narrow sea and cede bay of ice than vice versa.
Your best win condition is taking either 3 lannister castles and Moat cailin or 3 stark castles and riverrun.
You should treat riverrun as yours by rights, lannister is merely borrowing it and should kindly return it later.
Crackclaw point is surprisingly bad, take Moat cailin instead. I was really expecting ccp to be higher.
Don't go south but if you have to keep an eye out for starfall and highgarden. That being said though...still don't go south.