Italy will be the first to go

Six Reasons Why Italy May Exit Before Spain

1. Rise of the Five Star Movement
2. 44% of Italians view the euro negatively, only 30% favorably. That is biggest negative spread in the eurozone. In Spain more view the euro positively than negative, albeit by a small 4 percentage point spread.
3. A separate poll shows a mere 50% of Italian would vote to keep the euro if given a chance. That is the lowest percentage in the eurozone.
4. Italy is too big to bail
5. Interest rates have reached a point where Italy will struggle to roll over its debt
6. Eurozone Impossible Politics: The Bundesbank said there should be no banking union until there is a fiscal union. Angela Merkel said that there should be no fiscal union until there is political union. And François Hollande said that there should be no political union until there is a banking union.

Here is the bonus seventh point: Italy has enough gold reserves that it could avoid hyperinflation if it left the euro.

Read more at http://globaleconomicanalysis.blogspot.com/2012/06/six-reasons-why-italy-may-exit-euro.html#umsBab11WPKqd712.99

Update:
"We think that Italy, as opposed to Argentina in 2001 and Spain today, would survive a euro exit without big problems. Given the current public and political opinion in both Italy and Germany this scenario has a high probability to happen in the next 2-3 years."
--http://www.testosteronepit.com/home/2012/7/26/italian-euro-exit-why-it-might-come-in-2-3-years-and-why-it.html