The most important policy issue I see in the US right now where thing aren't currently going the way I'd like is immigration. It seems to me that we ought to be allowing much more immigration into this country, that this would give some net benefits to the US, and that it would be a huge boon for those allowed to immigrate.
At one point the US let many more immigrants in than it does today. But panic about foreign anarchists and concern that too many immigrants were from Asia let to a succession of laws restricting immigration. Despite the fact that the US has a far larger population nowadays than in the 1910s, we actually allowed more immigration back then in absolute terms.
The benefits to people who are coming to the US are pretty clear. They'll be a lot better off materially in the US, even illegal immigrants to who don't get the full protection of US labor laws. It isn't just that they're taking "our" resources, people from poor countries who move to the US benefit from our generally decent infrastructure and legal system, and genuinely produce more stuff than they would have in their home countries. Some people have estimated that this difference could amount to trillions of dollars to the global economy if everyone who wanted to immigrate wanted to.
You could argue that allowing the best and brightest to come to the US will rob their home countries of those same people. True enough, perhaps, but at the same time the remittances those people send home are by far the most effective form of foreign aid per dollar, and also far larger than all the other sources of foreign aid.
You could argue that immigrants steal our jobs, but that's just silly because the number of jobs changes with the number of consumers. Just look at Texas which has done very well in growing it's employment while people have been flocking to the state, to the extent that it's doing much better than most of the country. You could also argue that immigration drives down the wages of the those at the bottom of the economic heap and here you're on more solid ground. But the great thing about growing the economic pie is that even if a rising tide doesn't lift all boats by itself, redistributing gains is actually one of the areas that the government is actually pretty good at. If we're so much more worried about the livelihoods of native born Americans than about those born in, say, Cambodia that we would consider preventing the person from Cambodia from moving to the United States, well, instead we can just take some of that factor of ten or so more money that the Cambodian will make in the US, give it to the native born workers that he's competing with, and everybody will be better off. Doing something like that would force us to give up the pretension that we care about all people equally, but I think illusions aren't worth much compared to actual improvements in people's standards of living.
One could also argue that we should worry about diluting our culture or such - that too many immigrants might be more than we can handle. Since apparently 40% of the population of the third world would like to move to the US this is understandable, but that's no reason not to allow a factor of 10 or so more immigrants into the US each year. The US managed fine in the age of Ellis Island - not perfectly but certainly well enough.
So, who on the current list of presidential contenders score well by this metric? Assuming he gets the Libertarian Party nod Gary Johnson seems to be someone I agree with on this particular issue. I'll also say that Obama and Gingrich seem to want to improve things from where they are now. Conspicuously failing on this metric is Ron Paul, who despite his Libertarian leanings comes off as almost nativist at times. And Santorum and Romney don't do well, either.