City State of the Invincible Overlord is probably the best single city book, though it might not fit all settings. Probably my favorite.
Bluffside is actually probably a better city book than above, but has a lot of weird setting that that you probably will chuck out. Like dragon people and it's got a weird background (a city founded by Archaelogists?)
Freeport, besides the jokes, is actually a little scant on the city part. A lot of the book is actually extra rules and such. You really need Denizens of Freeport to flesh it out (though you can get that book pretty cheaply).
Bard's Gate is pretty good. But like Freeport in having a lot more extras when it needed more fleshing out (there's like 16 sections of the city, or something like that, but some only get 3-4 pages). But OTOH, it's extras are more useful, being 2 adventures and the Gods of the Necromancer Games setting.
Hollowfaust is pretty good, but somewhat Scarred Lands specific. Not too much. Small-ish book though. Same with their Sin City book (Shelzar). Both worth picking up cheap, though.
Though they are adventures, Lost City of Barakus & Grey Citadel do a pretty decent job of describing cities.
I like 7 Cities a lot, but the smaller towns are the more interesting ones, not the cities.
Shadizar for Conan is probably the worst I've ever seen. Glad I didn't buy it (traded for it).