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, leaving you with fewer opportunities to strike in a single round. Though often rare, they also are the most valuable Melee weapon in the game.
The super sledgehammer can be obtained at least two ways. You can go to the Old Town in the Hub and buy it from Jake or you can Rescue the initiate (also in Old Town) and get it as reward from Talus. A member of the Blades carries one. Moonshiners drop it when you meet them with a caravan. There might be further alternatives to obtain it also.
The super sledgehammer can be found in Navarro. It is located on the ground floor of the facility in the maintenance building east of the hangar. It is in the right-most of the two workbenches behind Quincy. He will not take kindly to you going through his stuff so it may require a sneak skill to obtain without alerting him and/or the base.