I'd say there are basically two groups of countries that concern me.
The first is the economic and military superpowers, now and to come. China is an increasingly potent force, and that's already bringing its interests into conflict with the United States to an extent. I just hope that the leaders of both have the wisdom to resolve their differences peacefully. The current US administration isn't capable of that, but thankfully it'll be gone next year: let us hope its successors have more sense. Russia is also a concern, because of its military power, mineral reserves and belligerent leadership. It might be an economic basket case, but it has to be taken seriously. India shows less belligerence in general, but much the same applies to it as to China, especially because of the ongoing rift with Pakistan.
The second is the Middle East. As the oil gets more scarce, conflicts there are only going to sharpen. The two main worries, I think, are Israel and Iran. Israel needs to stop treating the rest of the region with such high-handed contempt. Iran, meanwhile, is basking in its increased power now that the US has kindly taken out its main rival, and under that maniac Ahmedinajad it's pulling the Americans' tail. A dangerous game, IMO. Pakistan is a concern too: unstable, strategically important and nuclear. Not a particularly reassuring combination.
The second-rank powers - Britain, much of Europe - are less worrying in themselves, but still a concern. Less so now, maybe, since the folly of following the US into blundering around the Middle East, trying to bring 'democracy' by force is more apparent than it was a few years ago.
