Tungsten rods are the typical munition of choice in a "Rods from God" scenario.
Take a 10m Tungsten rod of diameter 1m.
It would have a volume of 10m^3.
A mass of about: 200000kg
Consequently, falling from a low earth orbit of 1000km, it would gain:
A kinetic energy of: 200000 * 9.81 * 1000000 = 2TJ.
Further to this, an impact of this magnitude would cause a huge nuclear reaction at the impact site releasing yet more energy.
The kinetic energy of this scales proportionally to the mass of the rod and the altitude it is launched from.
Tungsten is usually chosen because it has a melting point of 3,422 °C so a rod made of it would likely survive re-entry intact.
As an added aside, another benefit of using rods over meteroids is that the concentrated impact point will result in it being able to penetrate a target very deep in the ground, underground bunkers would not be safe.
Since the comments have exploded in a debate around the practicality of "dropping" something from orbit, the other option would be to slingshot it around the moon in such a way that the return path was a direct collision with your target. This would require about the same amount of energy as dropping it from orbit (it takes about whilst also raising your effective altitude to 400,000km.
This would give you an 800TJ impact from the aditional altitude alone.
You could also manufacture the rods in lunar orbit to further reduce the energy costs.