My favorites are Königsstein Fortress (just giant and impressive in every aspect, also overlooking a beautiful countryside) near Dresden and Heidelberg Castle (long a ruin but carefully restored just to the right extent - you can see the severe damage when it was blown up, but it's also worth to see it from the inside). Also, impressive in a different way, Ehrenbreitstein Fortress in Koblenz. It is a way more recent fortification, built when artillery was already way too powerful for the old fortification style but still had limited range. This way, if you see the Ehrenbreitstein from across the river it is a big fortress, but if you approach it on the only way an army would have attacked it, it's hidden in the ground and you can be grateful they made a fence so you don't actually fall in the first fortification trench (easily 15 meters deep). And it's a nice mixture: From the "enemy" side it's dark and powerful, from the "friendly" side you see it also was to show the beauty with different painting, light colors and the likes.