Here are my contenders. Roll 1d8 between:
1- Gazetteer 1: The Grand Duchy of Karameikos (TSR, B/ED&D, 1987)
2- La Cite Sainte de Laelith (Excelsior Publications, multisystem, 1991)
3- The Complete Masks of Nyarlathotep (Chaosium, Call of Cthulhu, 1996)
4- Ptolus: Monte Cook's City by the Spire (yes, already. Malhavoc Press, Dungeons and Dragons, 2006)
5- La France (Oriflam, Hawkmoon, 1991)
6- Les Dieux de Glorantha (Oriflam, RuneQuest, 1987)
7 - Genertela (Oriflam, RuneQuest, 1987)
8 - Shackled City Adventure Path (Paizo Publishing, Dungeons and Dragons, 2005)
If you make me choose, I'd say right now The Complete Masks of Nyarlathotep (which includes the Australian chapter not present in the original boxed set). The most complete, open campaign EVER, travelling between New York, London, Cairo, Nairobi, Shangai and more. You've got this and the Call of Cthulhu core book, you can play for years and years without needing anything else than dice, pencils and paper. It's that good.
Tomorrow, I simply could choose something else or roll the die.