Where was the worst tornado ever?

That is difficult to determine. The highest winds ever recorded in a tornado were measured on May 3, 1999 near Oklahoma City. However, most tornadoes don't get their winds measure so its entirely possible, even probably, that others were stronger. Candidates include the Tri-State tornado of March 18, 1925; the Xenia, Ohio tornado of April 3, 1974, and the Jarrell, Texas tornado of May 27, 1997.