It is hard to say which one was the "worst" as that is somewhat subjective.
The deadliest tornado season on record was that of 1925. On March 18 of that year the Tri-State tornado, the deadliest single tornado in U.S. history, killed 695 people. This death toll alone is greater than that of any other tornado season in the U.S. In all the 1925 tornado season killed 794 people.
The deadliest tornado season that does not owe its severity to a single tornado was the 2011 tornado season, which left 553 dead, with most deaths resulting from the Super Outbreak and the Joplin tornado. April 2011 had, by far, the highest number of tornadoes on record at 758. The month also produced the 2011 Super Outbreak of April 25-28. It was the largest tornado outbreak on record, spawning more than 350 tornadoes in just over 3 days including a 24-hour record of 207 tornadoes on April 27. Four of those tornadoes were rated EF5, marking only the second time that more than two F5 or EF5 tornadoes occurred in a single day. May 22 produced the Joplin, Missouri tornado, which killed 158 people and cost $2.8 billion in damage, the costliest single tornado in U.S. history and by far the deadliest in the era of modern forecasting.
The most severe tornado season on record was in 1974. That year produced a record 29 tornadoes rated F4 and 7 rated F5, most of them during the Super Outbreak of April 3-4, the most violent tornado outbreak on record. That outbreak produced 23 F4 tornadoes and 7 F5s in less than 24 hours. Despite the higher number of violent tornadoes, 1974 fell short of 2011 in deaths due to three extremely deadly tornadoes in 2011.
Spring is when the U.S. generally has the worst tornadoes.
1925. There were 794 tornado deaths in the U.S. that year, 695 from a single tornado.
The worst tornado damage of 2010 appears to have been in Mississippi, particularly from long-track EF4 tornado that move across the state on April 22.
The worst tornado was the Daulatpur-Salturia tornado in Bangladesh, it killed more than 1300 people but its intensity is unknown.
There are not publicly available tornado season forecasts. However, seeing as it is now late June, Kansas is probably past the worst of its tornado activity for 2013.
In a violent tornado the worst features are flying debris and suction vortices, small whirlwinds that have stronger winds than the rest of the tornado.
The worst level of tornado that can strike is an EF5.
Probably the worst, or at least the deadliest tornado in the world was the Daulatpur-Salturia in Bangladesh on April 26, 1989.
The worst tornado to hit Cowley County Kansas was the Udall, Kansas F5 tornado of May 25, 1955. This tornado killed 80 people, making it the deadliest tornado in Kansas history.
Unlike hurricane season there are not official limits to tornado season. However, generally tornado season lasts from lat march though June. However significant tornado outbreaks can occur at almost any time of year.
The worst tornado was the Daulatpur-Salturia tornado in Bangladesh, it killed more than 1300 people but its intensity is unknown.
The worst tornado of 2012 would probably be the EF4 tornado that hit the towns of Henryville, New Pekin, and Marysville in Indiana on March 2, killing 11 people.