Ruler controls appointments, government system, and has more control over diplomacy than anyone else. Ruler can also muck with taxes a bit. Mechanics wise, that's about it. However, Rulers can be contacted by anyone on the island, which is very handy (though not as important now with the new contact system), and control of message groups as well as innate respect to the position is very, very helpful in a less tangible sense.
General has only the powers he can take for himself, and assigning/disbanding militia. Handy for rebellions, but not much else.
Banker has control over realm taxes (and the ability to muck with the books), which can put a lot of gold in his pocket, but usually only once or a very small amount, relatively speaking, over time.
Judge has fines/bans/lifting of bans/control over the prisons, which adds up to a lot over time.
I'd say Ruler has the edge right now because of the absolute control over the government system, Ambassadors, and appointments to duchies, Judge second, Banker third, General fourth. Mechanics-wise only, of course.
Beyond that, Ruler has by far the best position in a less tangible sense. On the other hand, all of these pale in comparison to the power of a non-capital Duke, especially if that Duke has a Council position besides.
