My two votes go for Freeport and the Iron Kingdoms.
Freeport is just slightly off the cuff and sets a really nice atmosphere and tone for the campaign (DM dependant, of course). I just really like it. Unfortunately, however, i's out of print, so you'd probably have to find a copy on ebay.
The Iron Kingdoms also comes with its own tone and is slightly wacky, but a lot of fun. It's also covered in a lot more detail than Freeport and is still in print.
If I had to choose, I'd go for Freeport, but hey, that's just me.