Snarkhunter
Prince
The corruption model in the game is way overdone. Some government types are more susceptable to corruption than others, but you cannot escape it. I figure that republic, democracy, and monarchy are probably the lowest, and it gets worse from there.
Historically, western monarchies were extremely corrupt; it was the basic model of governance, as the central authority had comparatively few dedicated minions of its own to carry out its wishes. You could think of it as a franchise operation, in effect, with the franchisees being given license to exploit rather freely. This state of affairs persisted until monarchies evolved into constitutional monarchies, then slowly dissipated. (On the whole; 19th century French constitutional monarchies evinced a relatively high level of corruption until the Debacle of 1870, when everyone--well, most everyone--got tired of it for good.)
kk