I don't understand why Skingrad is winning for several reasons (assuming the cities somewhat resemble those in the games and that the hills around the castle are slightly easier to climb because lets face it, they aren't really that steep):
The terrain around is so flat and open, surrounding the city would be a snap.
The castle is separated from the city proper, and thus it is possible to completely cut the castle off from the rest of the city, which could lead to all sorts of problems like loss of organized leadership, large food stores in the castle no longer available for the main portion of the population in the city.
There is no river/bay near them so the above mentioned easy surround makes their supply situation all the more dire.
Also, with the city being cut in half, it would be possible for attackers to assault one side in force, take it and hold it using the bridges connecting them to launch raids against the other half, ect.
Now, assuming that there is somewhat of a naval presence in this world unlike the game, I would say Bravil by far. As long as they hold sea-superiority against the attackers, they could use a Pericles-in-the-Peloponnesian-War strategy by pulling everyone from the countryside into the city and keep their supply lines open on the sea, where (this is all assuming of course that Bravil is friendly with the IC, Anvil or any other small town on the Niben) they can get a constant stream of food whilst defending a small, rope bridge which if they felt was necessary, they could simply cut it down as a last resort and stall the enemy even more.
And lastly, the Castle is on the other side of the river, so as long as they have naval superiority still, they can keep the city and the castle connected by the bridges and if they need to, they can fall back across the river into the castle but have to abandon the city.
Of course, without naval superiority, they lose
Also, Cloud Ruler might be nigh impossible to take by assault, but there is no way out of that place and I can't imagine it can hold infinite amount of supplies.