For Mundane Items the Flail and Warhammer are the most damaging because they both deal 1d8 bludgeoning. I say these weapons over the other 1d8 weapons based on the fact that in the MM there are four creatures vulnerable to bludgeoning and the other mundane damage types do not have this trait.

   As @KorvinStarmast said, the Rod of Lordly Might, Flametongue and +3 weapons deal more damage on average than the others.