Just about any castle ruin, anywhere. Germany has no shortage of these, particularly in the central part of the country. Almost any castle that has not fallen into ruin probably looks the way it does today because somebody maintained and updated it throughout the ages to keep it habitable. There are plenty of castles that never faced the brunt of an invasion but later crumbled because they were not maintained.
But... if you're looking for an intact castle that more or less resembles something like it's orginal appearance with a minimum of latter-day elaborations... here's a few suggestions. By no means should you consider this a comprehensive list of any kind.
Marksburg and Burg Pfalzgrafenstein on the Rhine are both well known and need little introduction, as does Burg Eltz above the Mosel.
It's in an area few tourists will ever visit, but in the Waldeck region of northern Hessen: Felsburg above the small town of Felsberg (note the difference in spelling). The nearby town of Fritzlar is also a great untouristed alternative to over-touristed Rothenburg odT.
Marburger Schloss has been somewhat prettied up over the centuries (as have many of the castles mentioned here), but it more or less maintains it's original structure.
Burg Kronberg in Kronberg im Taunus... same. Never destroyed, but updated throughout the centuries.
Burg Breuberg over the town of the same name in the Odenwald region has undergone only structural repair over the years. It closely resembles it's original form.
Burg Zwingenberg above the Neckar River has some more modern structures mixed with the medieval fortifications.
Burg Trausnitz in Landshut survived completely undamaged into the 20th century, even avoiding the destruction of WWII, but then suffered extensively in an accidental fire in the 1960s. The structure was restored as close to the original as possible, but much of the decorative flourishes are gone forever.
Burg zu Burghausen in Burghausen is the "longest" castle in the world. Much of it is original.
In Franconian Switzerland (a region of northern Bavaria): Burg Pottenstein and Burg Rabeneck look more like thicked-walled houses placed on strategic heights, but I guess they qualify as "castles". Nearby Burg Rabenstein looks more classically "castle-like".
Burg Falkenstein is just one of the relatively unknown gems (to foreign tourists) of the Harz mountain region.
Finally, Veste Coburg, the ancestoral castle of the House of Saxe-Coburg und Gotha (agnates of which now sit on the thrones of Belgium and the UK), was largely rebuilt in the 19th century. But this huge structure is so interesting, and filled with so many weapons that it would be a shame to bypass. I think it's Germany's most interesting castle, personally.
Someone more familiar with Saxony than I can probably mention a few good ones in that federal state.
