If you don't want to read through my numbers, just skip to the middle of this post.
Here are some statistics nicely formulated by CivAssist (well, I added some information):
No FP, corruption 42g
Furuyama: corruption of 38g, not currently wasting or corrupting, would be built in 48 turns, would significantly reduce corruption in Epolenep, Regent Town, Manchai, TBD
Immo: 34g, not currently wasting or corrupting, 39 turns, Odawara, Corrino, En'Gha/R'yleh, TBD
Epolenep: 34g, wasting 3s, 3g, 46 turns, Regent Town (60 to 15% corruption), Manchai, TBD
Zojoji: 33g, wasting 1s, 1g, 23 turns, Osgiliath, Sto'vo'kor, Windurst, Pete, Bentropolis (in short, my province)
Odawara: 32g, wasting 4s, 4g, 25 turns, Corrino, En'Gha/R'yleh
Shorin-Ryu: 33g, wasting 2s, 3g, 20 turns, Corrino
Regent Town: 35g, wasting 6s, 4g, 27 turns, Epolenep, TBD (56 to 12%)
Corrino: 33g, wasting 2s, 4g, 28 turns, Odawara, Shorin-Ryu
Osgiliath: 30g, wasting 2s, 4g, 42 turns, my province (pretty significantly)
En'Gha/R'yleh: 33g, wasting 1s, 3g, 27 turns, Odawara, Regent Town, TBD
all others would take more than 50 turns
I think Odawara, Regent Town, and Osgiliath would all be good choices, both in the short-term and the long-term.
Odawara would help the corruption of three cities with significant corruption. It would also help corruption in Rome, and to a lesser extent, Iroquois.
Regent Town would help the corruption of 3 cities with significant corruption, two of them very significantly. It would also help the corruption in Rome and Babylon, and to a lesser extent, Iroquois.
Osgiliath would help the corruption of 5 very corrupt cities. It would also help the corruption in Zululand, and to a lesser extent, France.
Since Rome seems to be the next target, Odawara would help in future (war) expansions, Regent Town would be the second most helpful, and Osgiliath would take a while to have a significant effect on future expansion.
Regarding current territory, Osgiliath would be the most helpful, followed by Regent Town, and then Osgiliath.
Due to my recently acquired position, I'll have to go for Osgiliath (saves me the trouble of having to think

).