Worst tactic? I don't think there is one.
What usually the problem is: try to perform a maneuver which is great in theory, only to be beaten because your forces are not suitable to perform such maneuver.
Eg. double envelopement is cool, "Hannibal did it". But if you don't have enough men, it can happen that the enemy can destroy your centre before you start the maneuver.
One could say that the heavy cavalry charge of the french knights at Nicopolis (1396) was a bad idea. However, those guys were brought up with the idea that noone can stand before their charge. They fought numerous battles, where it worked perfeclty. According to their knowledge, this was the best way to attack the ottomans. It obviously didn't work, but they (at least the french) didn't anticipate that the ottoman cavarly will evade, and later return and surround them.