For my money you can't beat a swarm of self-replicating nanobots.
This has, of course, been tried once already here on Earth, with only partial success. A molecule got thrown together that happened to have the physical property of being able to create copies of itself. That in itself is already a fairly promising recipe for disaster, and indeed to start with that's what seemed likely to happen: the copies, and the copies of copies, multiplied exponentially, threatening to dismantle the entire world and turn it into more copies. The process stalled when it turned out that these replicators were not endlessly versatile in the types of raw material they could use, so all the immediately available usable stuff got used up, limiting the rate of replication. So, the copies turned on, and attempted to dismantle, each other for raw material. Random variation in the copies led to a situation where the best-equipped copies won and replicated, crowding out the less-well-equipped copies. This led to a de-facto arms race, externally unguided but effective because of the consistency of the selection pressures that determined the winners. It caused the resulting entities to grow more and more sophisticated, and to gain more and more abilities. A few billion generations later, the copies had got so sophisticated that entire colonies of them could move around coherently and exchange information with other colonies. The colonies themselves got progressively more sophisticated, until they were able to invent the internet, and to use it to ask each other how the job of destroying the world could be done more efficiently the second time around.
Sorry if you had different ideas about how we got to this point.
But by examining the mistakes made the first time around, the answer should be clear by now, and it is in two parts. First, design a self-replicating nanobot that's more versatile in the raw material it uses, so its exponentially multiplying copies can effectively eat the whole world without being contained in the bounds of their own ecosystem. Second, don't let them turn on each other: rather, get them to cooperate so that they make way for each other to spread outwards in search of more raw material. (This is where a bit of foresight is required in the design of the communication system.) Stick to this recipe, and we'll have the Earth teeming and homogenized before you know it.