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I would probably have to say the Egyptians for this. They have the phalangites for the street-fighting and axemen and superior defensive bowmen for the walls.
Hmmm... an interesting question. I would have to side with the seluciods. They got the pike men to defend the streets and gatehouse, and the roman legion imitations to defend the walls. Plus it never hurts to have a few elephants to charge a gate filled with the enemies men. 
Armoured Hoplites all the way! (just be sure to have a few Cretan archers and cavalry too)
Another one for the Selucids. They just have such a diverse selection for all of your seige breaking needs. Of course, any of the Romans are great too.
I had a battle where I was assaulting a town with 1 unit inside and the A.I. and the comp had 30 units of reenforcements. So my elephants break the gate and I capture all gates. Wiped out the town center and before the A.I. get's close to the walls I had my whole army inside. Unfortanately as soon as the units routed they headed for the town square only to get massacred on my huge infantry wall. Units outside the walls should route off the map not to the town center.
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