Relativistic Kill Vehicles
Seriously, if you are doing it any other way, you are doing it wrong. That includes Death Stars, Imperial Fleets, transphasic red matter quantum torpedoes, or any other silly nonsense normally found in the sci-fi canon. You just need to accelerate two ten-ton blocks of steel to 0.99 times the speed of light, aim them at your target planet such that they approach from opposite sides nearly simultaneously, detonate them a ways out so that they hit your target like birdshot, and then sit back and wait a few years for the dust to settle. Each 1 kilogram fragment (of which you just sent 20,000) would release an amount of energy similar to the largest nuclear bomb ever detonated. Virtually all advanced life would be wiped out, and a few years you can move in and take over your fresh new planet.
I bet you thought this post was going to be political, didn't you?