The super sledgehammer is the best melee weapon in terms of damage and knock-back effects. However, the greatest drawback of using it is that it knocks opponents very far away, giving fewer opportunities to strike in a single round. Though often rare, they also are the most valuable close combat weapon in the game.
The super sledgehammer can be obtained at least two ways; going can go to the Old Town in the Hub and buying it from Jake or helping rescue the initiate (also in Old Town) and get it as reward from Talus. A member of the Blades carries one. There might be further alternatives to obtain it also.
In Navarro, located on the ground floor of the facility in the maintenance building east of the hangar, in the right-most of the two workbenches behind Quincy. He will not take kindly to anyone going through his stuff so he has to be distracted or snuck past.
Moonshiners drop one when met with a caravan.
The last super sledge may also be found in the armory of the Enclave oil rig.