What is the worst tornado outbreak in history?

1925 Tri-State Tornado Outbreak was the worst

At around 1:00pm. on March 18, 1925, a tornado touched down over southeast Missouri and moved northeast through Illinois and Indiana. It grew to monster proportions and, in the three-plus hours the tornado was on the ground, it crossed three states, leaving in it's wake 695 deaths and dozens of destroyed communities.

Other, less severe tornadoes that day killed at least 52 people, bringing the total death toll to 747.