9. The Yuuzhan Vong
These aliens were kinda weird. They came on ships from outside the galaxy. They followed a religion that considered all inorganic technologies heresy (their ships and weapons were alive), and they saw pain as a holy experience. And they were pretty much the most badass warriors ever. The Vong invaded the galaxy and managed to conquer most of it, in an EU arc that lasted years and covered more than a dozen books. They were invisible to the Force, and they gave many Jedi some very epic battles.
8. Invasion Of Coruscant
Coruscant becomes the capitol of the New Republic, and the home planet of the second Skywalker-Solo generation. Its a world that is explored in countless novels and graphic novels, an endless font of stories. And as the capitol of the Republic, its a big damn deal when its invaded. The Vong manage to penetrate all the way to Coruscant, marking the high point of their invasion of the galaxy. They completely take over the world, then go about fixing it. Remember, Vong consider all inorganics inherently evil. So they alter the course of the planets orbit to bring it closer to the sun, then proceed to dismantle the entire city-planet and reshape it in their image. Its very, very epic.