Melee, it is debatable. The Daichi is not as powerful as other weapons but it does more damage per second than others. With the Knothole Island DLC you can buy a Master Katanas with 4 Augment slots, and with high level damage increase Augments such Katanas become more powerful than the Dachi. With the See the Future DLC you can acquire the Royal Sceptre, it has 4 augment slots and if equipped with Devastation Augment, Luck and Life Augment, Luck and Skill Augment and Lucky Charm Augment or any other damage increase augments it becomes by far the most powerful weapon.
The strongest gun is the Red Dragon does more DPS and longer range. The Enforcer, as with the Dachi is less powerful than a 4-augment Master Blunderbuss (DLC only) with damage increase Augments. The Red Dragon however remains superior to the Master Clockwork Pistol regardless of Augmentation thanks to its incredibly high DPs.