While I like photos of cities from an architectural standpoint, or by the way light will highlight one facade while creating dark canyons and shadows elsewhere, what I like most are photos that say something about the people who work and live there. Sometimes a photo will not have people in it to any extent, but if it says something beyond "here is some architecture" I will use it.
I like taking architectural abstracts also. They are fun because they can be some lines and colors that make a nice design on their own. But that is for a different portfolio.