City-State of the Invincible Overlord
The granddaddy of all RPG cities and holds up well over the decades.
Cities of Harn
Proof that a complete cities can be done in a dozen pages or less.
The newer versions are longer but are still have the smallest page count of any city supplement out there.
Cities by Midkemia Press
The best random encounter table ever written for city adventures. Also has a nifty Catchup table for what your characters does between adventures and a random settlement generator.