Choose the strongest city
Let's see, from a military and economic point of view
1. Bruma: small, remote (access can be easily blocked). The only advantage is steep terrain and cold weather. But the economy consist mostly of mining and is dependent on food imports.
2. Chorrol: nothing special to work with, except for the implied high living cost.
3. Cheydinhal: atl least one side is protected by a steep slope, not so much to say about economy excpet for the dunmeri immigrants, who are not so well off.
4. Kvatch: did had formidable defences, but not against a supernatural atack. It was most probably rich as it did have it's own arena.
5. Anvil: Bad fortifications placement, the docks are not protected and theres is a hill just nearby. The economy should be rather strong because of the sea trade but there is a number of abandonend buildings, maybe a crisis?
6. Skingrad: good defences and a strong, diversified economy.
7. Bravil: rather good defences because of the moot, but poor, rundown and with a
for a count
8. Leyawiin: fairly standart defences, should have a sea port, but this was srapped for whatever reasons (dev's lazyness?)
Overall I would say Skingrad.