No. 1 - Tambora, Indonesia, 1815
Tambora was the biggest volcanic eruption ever recorded. It was so big, in fact, that it canceled summer. That's right, no summer. So much ash was thrown into the atmosphere when Tambora exploded in 1815 that it effectively blocked out sunlight and solar radiation, reflecting it back out away from our planet, which started getting kind of chilly and cloudy as a result. Thus, 1816 became the year without a summer. Way off in Europe and the United States crops failed and people starved, while back in Indonesia 10,000 people were killed nearly instantly by lava flows and toxic fumes. The overall death toll from the explosion and resulting tsunami was 92,000 (not counting the death of an entire season).