Basically its a mech, that by the end of the series is pretty much capable of destroying galaxies, and has no real limit to its power, it could arguably grow to be a universe buster/reality warper if the need arose.
The death notes set of rules keep it from the most destructive weapon. Yes it can kill anyone, but you have to know their name and face and it takes time to write them down. It gets dwarfed pretty quickly by anything that can blow up a city, let alone worlds/galaxy/universe. Not to mention something like the infinity gauntlet that can warp reality.
Then again you may have an argument that you could do the most destruction while remaining anonymous. But the infinity gauntlet wins again.