The Baltic Coast
Although I am sensitive to issues of overcrowding (sorry, I haven't been to Rothenberg), I strongly felt that Lübeck was, by far, the best Hanseatic/UNESCO WHS city. The variety of special features (like Gäng, Rathaus, rivers, restaurants, luxury hotels (!), marzipan (allegedly less sweet, by local tradition), rail and air hub opportunities, and daytrip options, give it an advantage. Yes, it's jammed all summer, and the auto/bus roads have miserable traffic delays. But its very size delivers a net advantage in quantity of older buildings.
Let me take a moment to consider ... "wiederaufbauen." Especially in port cities, but really all over Germany, you almost have to assume that what you are looking at is actually a faithful recreation of what was once there, not the real thing. I noticed in Austria, plaques with "19xx Wiederaufbauen" are pretty common, but not so in Germany.
Lübeck, Wismar, and Straslund all have streets where, if you are lucky, there are three or four consecutive red-brick or stone buildings, often with a stylish script appliqué (contemporary) of the date of original construction. But they are surrounded by the same old, four or five story, smooth stucco over concrete block, faux-stepped gable, bright pastel paint, often with a dark or white perimeter (or around the windows) line, postwar "period" house.
This is much less common, for example, in Belgium, where "old town" centers are much more homogeneously "old", if completely surrounded by hideous new, low-rise urbanism. This contrast in France could be between Vitré and Dinan.
Although Wismar and Straslund were peaceful, small, and attractive (if NOT, undiscovered), I wondered sometimes how they achieved UNESCO status. For example, the Netherlands Hanseatic town of Deventer has a gorgeous, homogeneous, if compact, old center that ends up being much more rewarding - even though we had to walk through a vest-pocket Red Light district to get there. BTW, the church bells ring all night in Wismar, and there is Markt street noise at night ... ... . Particularly in Wismar, there is a strong and still visible historic Swedish culture presence. The seafood is very good. The churches and markt squares are exceptional in all three, but I vote for Lübeck.
If you happen to go to Wismar or Straslund or Rügen Island anyway (we did not have time for Rügen after all), I do want to endorse the Deutsches Bernsteinmuseum in Ribnitz. In two and a half floors, it provides a comprehensive story of amber. There are plenty of English labels. Much more is produced in Poland than in Germany. But I never expected to see a handful of amber nuggets from New Jersey! From a connoisseurship point of view, I will say that the one gallery of formal amber Decorative Art (meaning composite amber clocks, jewelry boxes, lamps, and so on), is not of the highest quality, such as you'd find in the MAK Cologne or the Louvre decorative arts building. It's all in a lovely gut-renovated cloister district that we did not have time to explore. However, it does NOT warrant a special one-hour or more drive, each way, if you're not in the area.
However, south of Wismar is the very attractive city of Schwerin. The old town has some of the same drawbacks mentioned above, but it is a very prosperous small city, with a major summer performance festival (that we missed.) The main attraction is Schloss Schwerin, and a very nice old town. Schwerin is not as big as Lübeck, but it is worth a detour.
Edit: Picturesque, cheap dining option in an historic old mill in Lübeck, way too far for day-trippers to eat in, Alte Mühle, specializing in Flammküchen, http://www.altemuehle-luebeck.com/ . Look for Gänge, beside the river, nearby while walking back to the center.