All things considered, I'd probably go with Pakistan, given that they have the double-whammy of nukes and widespread extremist activity. If I had to pick which conflict is most likely to result in a nuclear exchange, I'd put a major terrorist attack in India by Pakistan-based forces ahead of anything involving North Korea or Iran.
Not to mention the fact that they are already partially responsible for the proliferation of nuclear technology to North Korea, Iran, and Libya.All of the US, China, and especially Russia can be major threats to regional safety, but always seem to let cooler heads prevail before things escalate toward all-out war between major power blocks, or nuclear conflict.
