The German part of this tentative site was reduced from the original six locations to three in the final nomination. And rightly so in my opinion. The remaining ones are: Bad Kissingen, Baden Baden and Bad Ems. I agree with Els that Bad Ems is not really impressive. So my review will focus on the other two German sites.

As in most of the other spa towns, the hot springs in Baden Baden were already used by the Romans. The remains of the Roman baths can be visited below the market square. The boom in the 19th century is closely linked to the granting of the casino licence. In summer, the high society frequented Baden Baden: Europe’s nobility, but also artists, composers and writers. Especially the Russians. Dostoyevsky fell out with Turgenev here. And Nikolai Gogol wrote about Baden Baden: "I only wanted to stay three days, but for three weeks now I can't break away. No one here is seriously ill. Everyone comes here to amuse themselves."
The most important spa buildings are the Kurhaus with the casino in the right wing and the Trinkhalle (pump room), both in neo-classical style with Corinthian columns. This is also where the Lichtentaler Allee begins, a two-kilometer-long footpath through an English garden along the river Oos. It is the usual promenade of the spa guests, you walk along typical examples of 19th century architecture: luxury hotels, museums and the theatre. A stroll through the villa district on the slopes above the park is also nice. The wealthy lived here in the 19th century and still do today.

I have visited Baden Baden several times, also long before the Great Spas of Europe were added to the T-list. Usually, the Friedrichsbad was my main destination. The Friedrichsbad is an irish-roman thermal bath, opened in 1877 and built in Neo-renaissance style. In most spa towns, the historic baths have long ago been replaced by modern sauna complexes with outdoor pools etc. Baden-Baden also has such a modern ‘sauna landscape’. But in the Friedrichsbad you can enjoy bathing tradition in the authentic layout from the 19th century, which in turn was inspired by the Roman Caracalla baths. I highly recommend it, even for those who are not big fans of sauna. The hottest room is 68°C, so not as hot as usual in a sauna. The body is stepwise warmed up and then cooled down again in a sequence of hot air baths (the Irish part) and steam baths and pools (Roman). The bath is textile-free, usually for men and women together, but there are also days for separate bathing.

I visited Bad Kissingen in 2016 and it was a pleasant surprise. The architecture and the townscape are different from Baden Baden and the Bohemian spa towns. The core of the spa district is a complex of four connected buildings: the Regent’s Building, the Arcade Building, the Brunnenhaus (pump room) and the adjacent Wandelhalle. Two of them, the Regent's Building and the Wandelhalle, were built only at the beginning of the 20th century. The whole complex is a mixture of different architectural styles: Biedermeier, Neo-Baroque and Art Nouveau. The photos show examples of the Art Nouveau elements, the interior of the Wandelhalle (photo left) and the Green Hall in the Regent's Building (photo right).
I liked the Wandelhalle and the Brunnenhaus best. Long rows of columns divide the interior of the Wandelhalle into three naves, giving it the appearance of a basilica. The Regent's Building is actually not open to the public, it is only for festivals and events. But when I arrived at the tourist information, a guided tour of the spa district was just starting, also including the interiors. The other buildings are freely accessible and you can also taste the mineral water, but only at certain hours in the morning and afternoon.

The Great Spas of Europe are a worthy candidate for inscription on the World Heritage List. As for the German part, two sites would be sufficient in my opinion: Baden Baden and Bad Kissingen.