What are the united state's worst tornadoes?

"Worst" is a subjective term, but for these purposes death toll is probably a good proxy. So based on that the worst tornadoes in U.S. history are:

  1. The Tri-State tornado of March 18, 1925. This F5 tornado killed 695 people and injured over 2000, making it the deadliest U.S. tornado by far. It tracked across parts of Missouri, Illinois, and Indiana, destroying several towns. 234 people died in Murphysboro, Illinois, the most in a single town in U.S. history. The tornado tore a damage path 219 miles long, was on the ground for three and a half hours, and at times traveled at 73 mph, making it the farthest traveling, longest lived, and fastest moving tornado ever recorded.
  2. The Great Natchez tornado of May 6, 1840. This F5 tornado struck Natchez, Mississippi and Vidalia, Louisiana, killing at least 317. Most of the deaths occurred in boats on the Mississippi River. The actual death toll was probably higher than 317 as many bodies likely floated down the river without being counted. Despite the extreme death toll, only 109 people were injured.
  3. The St. Louis-East St. Louis tornado of May 27, 1840. This F4 tornado tracked across parts of St Louis, Missouri and East St. Louis, Illinois killing at least 255 people. Much like in the Natchez tornado more people likely died, but were carried down the Mississippi river before the could be counted. With adjustment for inflation to modern values this tornado caused $536 million in property damage, making it the tenth costliest in U.S. history. It is interesting to note that the three deadliest tornadoes in U.S. history crossed the Mississippi River.
  4. The Tupelo, Mississippi tornado of May 5, 1936. This F5 tornado obliterated a large portion of the town of Tupelo, killing at least 216. There is some uncertainty in this death toll as well as little effort was made to count African-American fatalities due to the racial policies in the South at the time.
  5. The Gainesville, Georgia tornado of May 6, 1936. This F4 tornado, part of the same system that produced the Tupelo tornado the previous day is reported to have resulted from two tornadoes merging. The death toll was at least 203. In one location the tornado collapsed a multistory building killing 70, the highest tornado death toll in the U.S. in a single building.
  6. The Woodward, Oklahoma tornado of April 9, 1947. This long-tracked F5 tornado swept through several towns in Oklahoma killing 181. It was part of string of tornadoes spawned by the same thunderstorm that claimed over 200 lives in Texas, Oklahoma, and Kansas.
  7. The Joplin, Missouri tornado of May 22, 2011. This EF5 tornado developed rapidly just outside of the city of Joplin and swept through it. Despite advance warnings 158 people were killed and over 1,100 injured, making it the deadliest tornado to hit the U.S. since the 1947 Woodward tornado (which occurred before tornado warnings had been invented) and the first U.S. tornado to produce more than 100 deaths since 1953. Damage from the tornado totaled $2.8 billion, making it the costliest tornado in U.S. history.
  8. The Amite, LA-Purvis, MS tornado of April 24, 1908. This long-tracked F4 tornado tore a path across parts of Louisiana and Mississippi, killing 143. More than half of these deaths were in Purvis, MS. The tornado was part of the Dixie outbreak, which sawanother very deadly tornado kill 91 in the area of Natchez, Mississippi.
  9. The New Richmond, Wisconsin tornado of June 12, 1899. This F5 tornado struck New Richmond just as tourists were visiting the town for a circus, which likely contributed to the dead toll of 117. A number of buildings and even a save were picked up and thrown by the tornado.
  10. The Flint, Michigan tornado of Jun 8, 1953, This F5 tornado stuck the Flint and Beecher communities of Michigan killing 116. Of those deaths 113 occurred along a 4 mile stretch of its 19 mile long damage path. The day after this tornado the same system produced an F4 tornado that stuck Worcester, Massachusetts and killed 94, the highest tornadic death toll in New England history.