I will have to go with Kvatch, for the same reasons everyone else has stated. The fact that Kvatch is on a plateau that could be easily defended gives the city an alternate way of providing food. The Imperial City itself would be the second easiest, but is vulnerable to disease. The sewer exits would have to be sealed to defend it properly. If Lake Rumare is just a large Lagoon with brackish water, a siege could end very quickly in favor of the attacking army. Bravil has the same weakness as the IC, and a criminal element that could be easily bribed. Bruma is easy to isolate and starve. The other cities are vulnerable because of bad location. Skingrad,Chorrol, and Anvil have too much high ground that could be used by an invading army.