I don't agree with people voting Tywin. His approach may hold together a single kingdom built on serfdom / feudalism for a time, but essentially being a dictator operating from behind the curtains of power never works out in the long run. Especially when that person is so ruthless they care more about glory, pride, and name than anything else. I think this was shown in the books / show.
I voted Ned Stark. While his "honor and justice above all" approach lost him his head in the books / show, this would actually work in a democracy like ours given the right leader (not Jon Snow....). He was also noble and, while not a scholar, wasn't an idiot either.
The rest are all fascinating to think about but would fail for some of the same character flaws that ended Twyin and Stannis.
Bobby B, I mean, cmon. And Bran really doesn't come off as a leader at all.