Which disaster would cause the most global damage?

Yellowstone and the asteroid would have similar results, but the asteroid would probably kill a lot more life immediately, if not all of it. The shock wave alone would be damaging all around the world, not to mention the resulting earthquakes and tsunamis that would take care of the survivors. After that, a cloud of debris would encircle the world and block out the sun, killing plants and freezing the planet. It could last for months or even years, enough time that anything unlucky enough to have survived the rest would starve or freeze. Yellowstone would be on a smaller scale, directly destroying much of the Northern US and Southern Canada, but the ash cloud would devastate crop production worldwide.