The eruption of Tambora in Indonesia in 1815 killed the most people. It was a huge eruption that sent ash into the stratosphere that then spread around the world. World climate was noticeably cooler the following year, and in places it was called “the year without a summer”. Closer to the eruption itself thousands of people were killed, and due to the destruction of crops, disease, contamination of water, etc., tens of thousands more died in the years that followed.
Overall about 92,000 people died as a result of the eruption. Back in 1815 there wasn’t much news coming out of Indonesia to the western world but if it were to happen today it would definitely have been a big deal.