The May 4, 2003 outbreak was very severe in Missouri (F4 at Kansas City, F3s at Stockton and Pierce City), but I highly doubt it was the worst outbreak in Missouri history (in terms of significant tornadoes, not just sheer number of tornadoes). Here are a few other candidates based on NCDC data and Grazulis' Significant Tornadoes:
1. May 27, 1896: St. Louis Outbreak
Outbreak for Missouri and Illinois devastated parts of St. Louis. Of the 305 killed, 255 were in and around St. Louis, Missouri. There were 1000 injuries and $10 million in damage for St. Louis alone.
2. May 8-9, 1927: Poplar Bluff Outbreak
An F4 tornado ripped through Poplar Bluff, Missouri killing 92 people. A total of 217 would die from 17 major tornadoes across Missouri, Arkansas, and Texas.
Gabe