ANTIMATTER
The most explosive substance known to mankind, when antimatter comes into an equal quantity of antimatter both are annihilated, as in, neither exists anymore. You can probably see where I’m going with this. Create an earth sized amount of antimatter, and bring the 2 together. This is likely to be difficult, and there are many ways to move objects, which shall be covered in part 3. It shall be difficult, since nothing can touch antimatter without decreasing the amount of antimatter and vanishing in a brilliant flash of light, which is another side-effect of matter and antimatter colliding. Then, just sit very far back and watch the fireworks. Take a video camera too, it will be that spectacular.

A second method of planetary destruction with antimatter is to get a significantly smaller amount, and blast the earth to fragments, enough so that it cannot reform under its own fragments gravity. For this you will need about 25,000,000,000,000 tonnes of antimatter.

Problems with antimatter include the storage and manufacture of it. Storage due to the fact that you cannot touch it, and manufacture since at current technology levels you can only manufacture it a few atoms at a time.