d20 modern help

[Nosta]

I need to know what melee weapon among the d20 modern books has the highest damage dice

[Ashtagon]

As with 3.pf, melee weapon damage pales in comparison to Power Attack.

However, the reaper laser scythe does 4d10 damage as a baseline. It's PL8 and only available for mecha though. Alternately, the beam sword does 2d8 fire damage, also PL8.

These answers are a bit meaningless without knowing what campaign is being run though. d20m is a very modular system, and it was written with the expectation that the GM would veto various sections (or have that veto exercised for him by choice of pre-designed campaign setting).