Cloud Ruler I think would not be a great pick actually. Look at the garrison, not a single Blade has a bow. Thus making them trapped in a cage, with few resources. They could be starved until they all died or opened the gate themselves, ruining it's tactical advantage.

Skingrad would also be poor to me because the castle is separate from the city, making the city hard to defend. Same with Anvil.

Leyawiin, Chorrel, and Cheydinhal would not be bad.

Bruma has an advantage because it is on the hill... but its flat on the other side, so people can just go to the back and it loses the advantage.

Bravil is actually on an island and would be easily defensible... just burn the rope bridge and you're set.

But I voted Kvatch... strong walls, strong people, and Savlian Matius. And its on a really big mesa.

Cloud Ruler Temple and Kvatch. Both have only one enterance, are on hills where they can see everything for miles (probably), and have good warriors (Savlian Matius and the Blades).

Or die trying...

Mara

Kvatch is my choice, yeah they got owned by the daedra. but do you really expect any city to be able to fight off multiple oblivion gates opening right in front of the gates in the middle of the night when just about everybody is asleep. with the siege crawler capable of taking down any city gates and the river of daedra flowing from each one of the gates no city could withstand that assault.

while kvatch could easily take the fight to the enemy outside of the gates im not so sure it would be easy to fight the enemy inside the gates.
skingrad on the other hand has balconys all over the place which could hold archers and spellcasters and it has a much higher population than some other citys so theres more people to fight off the enemy.

One point about Kvatch / Skingrad they have both been sacked at least once before.
Anvil has also once been burned down but that was after it lost to a superior naval fleet.
The other cities I'm not sure of but my point was, Kvatch / Skingrad not reliable on the defense scene.