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 8 others from the other books (as noted by Medix2). 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.