Sofia is not a beautiful city and is not well adapted for modern tourism. Especially when put in contrast to Bulgaria's other main tourist destinations, the Black Sea Coast and Plovdiv, Sofia lacks a compelling reason to visit. If you are here though, then I outline what there is to see, and where to see it.
For a curious traveler, Wiesbaden offers something that most German cities do not: a beautiful old town. In contrast to its neighboring cities of Mainz and Frankfurt, Wiesbaden offers an entire city of specacular vistas and 19th century atmosphere.
Basel is a charming and almost provincial counterpoint to the other great cities of the Rhine. Come visit for a beautiful but relaxed atmosphere and good food.
Yes, Siena is what I would describe as "Peak Europe." It doesn't get better than this. You can think of Siena as a smaller and more subdued version of Florence. Florence overflows with tourists, who crowd vast avenues lined with marble-clad palaces. In contrast, Siena awaits a smaller crowd meandering through alleyways of sun-baked brick.
The primary consideration on whether or not to visit Florence is the kind of experience you expect. Modern tourism thoroughly colonized the city, and tourists will vastly outnumber residents in the city center. However, the city offers an unparalleled opportunity to experience art and history in its original context and makes for one of the best destinations in Europe.