I'd say Schenkenseebad Schwäbisch Hall is on places 1-3..
And then, to mix it up a bit, you might try once Rothenburg ob der Tauber, the outdoor public bath, if they still have the FKK unit. But I'm not sure, I'm from another part of our country.
Anyway, how far are you willing to travel to visit good places?
Some might be worth a 2-3hours trip here, if that's ok.