It is hard to say. I first thought Robin (and voted her), but now I've thought of it, I'm not sure. Robin is definitely one of the more sensible of them, but she is generally too passive - she wouldn't be as quick to step up for a key issue as most of the others. And she dislikes the government. It could end up as quite the anarchy, though I don't know if she has some hidden political compassion that would make her try to prevent it.
Chopper would definitely the one with the best intentions, but he is too young, naive and ignorant of the world, so even if I voted him, he would just end up being manipulated by his peers and become more of a figurehead. Not to start a real life politics discussion, but if I was American, I would probably vote for him in hope of improved healthcare.
If I was a chauvinist female, I might have voted Sanji just because of all the new exclusive rights my gender would have. Even though I believe Sanji stated back in Alabasta that he doesn't understand politics…
Franky and Brook are weirdos - it would be hilarious to follow their escapades if they had a career in politics, but I just cannot take them into account if I have to answer seriously. Luffy is a retard, and while Zoro is smarter in some areas, he would probably be a retard as well when it comes to politics.
Back we have Nami and Usopp who would, with some exaggaration and prejudice from my part, be more or less the only realistic politicans. I'm not really a capitalist so that completely leaves out Nami. On the other hand, a Nami-governed country would probably not be capitalist but instead a caricatured socialist country with all inhabitants being public employees or on welfare payments so she could easier regulate how many money they would receive and how many the government could keep to itself (or herself, she would probably cheat and manipulate the others). In Nami's version, that's not any better.
Usopp is all around a decent guy with some sympathetic stances, and as others have mentioned, he has the gift for making speeches. Of course he has a few problems: He is a bit too young, he could be too afraid to act on his word, and he is liar. Not that I like the latter flaw at all, but I'm almost getting used to it with my country's current government, so if Usopp just kept one or two of his election promises, I could maybe put up with it. But still...
I don't really know... Can't I vote for an Usopp/Robin coalition?