I'd go for climate change. It's a pernicious problem, because on the one hand, it seems as though it's a distant one: though the warming we see is very, very fast by geological standards, it is quite slow by ordinary human standards. It's easy to think that there will be time to deal with it. But on the other hand, it's a 'back-loaded' problem: like serious weight gain, or serious debt, or drug addiction, the initial stages seem like no big deal; it's only in the later stages that one can see how deep the hole is that we will have dug ourselves into. And of course, there are many, many voices downplaying or even denying the issue because of fears that solving may hurt them economically. But it's worse with climate change, because even if we stop emitting greenhouse gases, warming will continue for many years, due to gases already in the atmosphere.
As to how we should tackle it, we first need to agree on mechanisms to include the costs of carbon pollution into our economy. As it is, anyone can basically use the atmosphere as a free dump for most combustion by-products. This is a huge subsidy for all fossil fuels. Second, we need to accelerate the development of renewable energy sources. It's already impressive; the costs have been dropping fast, and in some places are already competitive with fossil fuels. And renewables are now being used in over 80 nations, with growth rates of 30% and more per year. Yet there are urgent needs: how can we more efficiently store energy, so that the intermittent nature of sun and especially wind can be 'smoothed out' to allow us to rely on them to a greater degree? How can we best structure delivery of energy? How can we use less without compromising our economies and life choices?