It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked. (Gold)
It has the ability to reconstitute its entire cellular structure to transform into whatever it sees. (Diamond)
Ditto is everything. Not only can it transform into any Pokemon - it can transform into anything it wants, organic and non-organic things alike. Any Pokemon on this list? Ditto can become them. It can become whatever kind of Pokemon theyr'e weak against. It can become - theoretically - anything that exists. The moon, the Earth, Arceus, etc.
An organized army of Dittoes could take over the planet if they wanted - so could a lot of Pokemon, I guess. But the difference is that Dittoes could do it without anyone ever noticing. It could replace your family and your friends without you realizing it. Eventually, a Ditto could replace you. They could replace us all.
Unless...they already have.