In my story(setting--modern day), a group of terrorist got one nuclear device from North Korea(50-100 kiloton). Their goal is to destablize global economy so the common people in first world nation (include China and Russia) will lose their job and bank saving, causing something that looks like the Great Depression, but global. Where should they strike to accomplish that goal?
As far as I know, you cannot bomb Wall Street and crash the economy any more than you can destroy the internet by plugging out a computer. Turning Fort Knox into a pile of rubble would also not suddenly make the United States a third world country - the gold would still be federal property, even in its molten, irradiated state.
I can think of a few potential targets that could destabilise the world economy as a side effect, and none of them are particularly creative.
That would certainly start a war. You can detect the origin of the nuclear material from the fall-out; within a couple hours it would have been identified as North Korean. A declaration of war would soon follow, and if the right people in important seats screw up the diplomacy, it would become a world war. Which would, in turn, wreck the global economy.
Of course the Pentagon itself was designed to withstand nukes and the attack itself would not kill many more people than 9/11 did - it is the principle that matters.
Jerusalem, Rome, or Mecca
If one billion people became a bit depressed at once because their most sacred religious leaders or sites were blown to bits, it will impact their spending habits. Even just uncertainty over their potential changes in spending would upset the stock exchange, make currencies rise and fall against each other, and while it's possible some countries end up profiting off it, in the short term there would be trouble across the board.
This too would mostly be indirect. Taking out Berlin would quite devastate German economy, which is the powerhouse of the EU, which itself is a global player. But here it is more the idea that a safe, non-offensive city leagues from violent wars and completely without controversy: the idea that such a place could be turned to ash in a day because some terrorist group/nation wanted it; that would upset people everywhere. And again, upset people in large numbers can wreck an economy that's based on speculation the way our global economy is.