I think it's difficult to carry on a discussion about best or worst without taking into account the context of certain battlefield decisions. A frontal attack on today's battlefield, for instance, is nothing less than suicidal and stupid. But during the Civil War or Revolutionary War, this was a sound tactic.
My nomination goes to General Antonio Lopez de Santa Anna. Talk about ******* up a wet dream. His forces had momentum after the slaughters at the Alamo and Goliad, and he had Sam Houston on the run. At San Jacinto, Santa Anna had every tactical advantage; he outnumbered the Texans 2:1; he had better artillery; he had professionally-trained troops; he had everything in his favor. Santa Anna's problem was his arrogance, and that arrogance led to a fundamentally stupid decision: he never posted pickets at San Jacinto. He had no idea that the Texans were sneaking up on him until it was too late. He lost the Battle of San Jacinto in 18 minutes with catastrophic losses.
While anyone can have a bad day, I consider Santa Anna pretty stupid because he again had superior numbers during the Mexican War when Winfield Scott went to Mexico City. Santa Anna had 16,000 troops under his command compared to Scott's 7,000. Santa Anna lost approximately 3,000 troops after a series of battles.
My argument for Santa Anna being the worst is that his slaughter of the Alamo defenders and what would be considered war crimes by today's standards emboldened the Texans. They began their charge at San Jacinto with "Remember the Alamo!" and "Remember Goliad!" His miscalculation of American military capabilities during the Mexican War cost Mexico all of its land north of the Rio Grande (although Texas had already won its independence) resulting in what is today California, Nevada, Utah, Arizona and most of New Mexico and Colorado being annexed by the United States. So Santa Anna's defeat was costly for the Mexican government.
That's why I pin the tag on him: stupid tactics, dropping the ball when he had the tactical advantage, inciting resistance rather than winning support from the local population, and losing big when the stakes were high.