There are two considerations: Debt to GDP ratio and country size. Here's a picture of the Eurozone:

The diagonal lines are at 75%, 60% and 50% Debt-to-GDP ratio. 75% and 50% are the 1st and 3rd quartiles of the Eurozone, and 60% is the well-known Growth and Stability (suicide) Pact threshold.

Smaller countries are most vulnerable to a market run because it take less capital at risk to manipulate their debt market. Countries with a higher Debt-to-GDP ratio are also most vulnerable because the threat of default can be talked up more credibly.

It looks to me like Belgium and Portugal are next after Greece. Italy is an attractive target, but too large. Austria, the Netherlands and Ireland would be a second tier, of which Ireland has already capitulated and both bailed out their financial sector and engaged in "model" fiscal austerity.

Maybe looking at deficit would make Spain look more vulnerable, but on Debt and GDP, it is only a 3rd tier prey for the wolfpack.

By laying out pros and cons we risk inducing people to join the debate, and losing control of a process that only we fully understand. - Alan Greenspan