OK, as requested, I'll limit my comments to personal experiences.
The place I've visited most isn't technically a Kurort, but an all inclusive resort on the outside of the medium-sized town of Erding. This is, of course, the fabulous Therme Erding, the world's biggest spa resort. Love it, love it, love it. Erding itself is pleasent enough, if nothing particular special. Should you desire, you can easily catch an Sbahn into Munich.
Probably more of what you have in mind is Bad Kissingen. It's kind of like a Bavarian version of Baden-Baden, but quieter and with a fraction of the bling boutiques. I didn't visit the main Therme in town, mainly because my hotel had a perfectly adequate set-up. The town's Kurgarten is particularly large. No lake, but Bad Kissingen is in a (low) mountainous region of northern Bavaria. The town also has a high-class casino (high class, as opposed to the shady little Spielotheke you find all over the country), but I didn't check it out.
I only visited the Therme in Bad Staffelstein and not the town itself (what I saw looked pleasent if unremarkable). It's in an attractive valley near several interesting towns (Bayreuth, Bamberg and Coburg). The Therme could use an update from the 70s style decor, however.
Finally, I'll mention Krystal Palm Beach, which is another large indoor waterpark/Therme, although not nearly as large as Therme Erding and it also doesn't have attached lodging. It's in Stein, a close suburb of Nürnberg. The Therme section has a very nice selection of themed saunas, although don't bother if you're not comfortable with public nudity.