None of them because:
Bruma: Cold and backdoors, that and the doors seem weak and the mountains could provide a trebuchet with easy ways of destroying the city from above
Skingrad: The last holding place would be the castle: Destroy the bridge -> Win the city
Kvatch: there is a reason it fell for the Daedra, and if they take up the walkways the only way is digging out of the city
IC: a total of 6 entrances and the sewers could be exploited
Anvil: Close to the sea could cause a blockade of supplies, if combined with a land assault
Leyawiin: 2 entrance ways, already flooded, the castle is really close to a gate
Chorrol: Siege the city, finish up the digging in the nearby cave
Bravil: Diseases would spread really quickly during a prolonged siege
Cheydenhal: the counts son and the gangs could make a diplomatic way of gaining the city