Iowa. Only six electoral votes, no Senate seat up for grabs, and no egos (as in "Paul Ryan" or "Scott Walker") at risk. Nevada has as many electoral votes, but it has a Senate seat up for grabs. Wisconsin and Iowa vote largely in tandem -- and when they voted differently for President in 2004 they were both decided by less than 13,000 votes. But it has a Senate seat at risk.
To be sure Iowa puts seven electoral votes at risk, if you include one of the electoral votes in Nebraska because it is in the same TV market as most of western Iowa. (If you don't know how Nebraska allots its electoral votes you need a little lesson).
So far, Obama has been more likely to make ad buys in the peripheral out-of-state markets than Romney. For example, as of a week ago, Obama was up in the Burlington-Plattsburgh and Portland, ME markets, which serve parts of northern New Hampshire, while Romney was only up in Boston to reach southern New Hampshire, where most of the people live. Similarly, the Romney campaign hasn't been up in Omaha in recent weeks, while Obama has (though some Romney PACs have been up in Omaha).
It will be interesting to see if this changes in the upcoming weeks. Romney is supposedly starting a huge ad blitz tomorrow.
I don't think the campaigns will pull out of Iowa first. New Hampshire would likely come before that, since it's expensive to advertise in the Boston TV market.