Below is a very telling info-graph that scales countries around the world to their debt level compared to GDP. As you can see Japan and a number of European countries (Greece, Italy, and Portugal) are the largest out there.
It is important to note that a high debt to GDP ratio is obviously negative for countries there are a number of other countries like Brazil and Argentina who are in trouble and have much lower ratios.
Info-graph courtesy of howmuch.net.