You know, narrowing it down to just German regions is tricky because a lot of the famous 'German' werewolf stories come from places that aren't Germany anymore. The whole Peter Stumpp case, the 'Werewolf of Bedburg,' happened near Cologne, which is solid ground. But a massive chunk of the lore I've read comes from the broader Germanic cultural sphere, like those insane woodcuts and pamphlets from the 16th and 17th centuries.
That said, if I had to pick a region within modern borders, I'd lean towards the areas with deep, dense forests and a history of isolated communities. The Black Forest seems like an obvious candidate—all those Grimm fairy tales and that sense of a dark, impenetrable wood feels like perfect werewolf territory. But honestly, I think Bavaria and the Alpine regions might be even richer. You've got traditions of the 'Wolfssegen' (wolf blessings) and shapeshifter legends tied to the Wild Hunt, which often blurred with werewolf myths.
Hesse also pops up a lot in the older scholarship I've skimmed. The lore there often intertwines with witchcraft trials, where accusations of lycanthropy weren't uncommon. It feels less about the cinematic monster and more about a societal fear of the animalistic, the outsider. So the 'riches' depend on whether you're looking for historical trial records or the folktales that survived in collections. For the latter, I'd say the south and southwest have the edge, partly because the terrain itself feels like a character in those stories.