Well if we're going to use some initial criteria it would have to be someone who is in a strategically important part of the world who is a major influence on 1) energy, 2) economic policy-making, 3) a criminal-state nexus, or 4) social movements.
1) In terms of who can leverage their military and political capacities to impact the world's energy networks, that would of course be President Obama.
2) Economics, well everyone's talking about the Eurozone crisis and front and center is Angela Merkel and her attempts to reign in unruly Southern Europe.
3) There isn't currently a major conspiratorial network with a single leader that I know of that is able to wreak havoc on a global scale. After jihadist militants in Afghanistan and Pakistan, the closest would probably be the cartels in Mexico and their ability to burrow into the state and affect policy in the US, but that remains a regional, potentially trans-Atlantic crisis. Plus there's no clear leader in that conflict.
4) Finally with regard to social movements there's no real leader who's come to channel the hopes and aspirations of a global generation, or create a revolutionary ideology like Marxism. The Arab Spring, OWS, and other 2011 protests remain rudderless in terms of creating a coherent alternative to neoliberal globalization, and there's no single figure who has come to embody the hopes and dreams of those movements.
The rest--China's current leader, Putin, Netanyahu--may have power, but it doesn't look like they're going to exercise it in any way that's going to disrupt the order of things. They're known variables. Though I'm holding my breath on whether Netanyahu may yet cause a major global crisis by attacking Iran.
My vote though for now goes to Angela Merkel since she is in a crisis situation that is affecting the rest of the world.