Southern Germany, Central Austria, & Northern Italy

I don't know anybody who says Zurich is a MUST SEE. Not that there is anything wrong with it. Luzern might be a bit more interesting scenically. OK, my list.
Italy: Varenna on Lake Como, Ortisei in the Dolomites, Venice
Austria: Salzburg
Southern Germany: Munich, Neuschwanstein Castle in Fuessen, Nuremberg, and I suppose Rick would get mad if I didn't say Rothenburg ob der Tauber, seems like everybody's got to go there on their first trip.

Nothing wrong with Zurich, but I would not put in high on my list of places to see in Switzerland. Luzern is a definite must see.

I lived in Augsburg, Germany for four years and loved the Fussen, Oberamergau, Garmisch area of Germany. Also, the Zugspitze is goos, of course, if you are going to Switzerland and go up the Youngfrau, you might not want to do the Zugspitze. Interlakken is a great area for a couple of days. Also, I loved Innsbruck, Austria as well as Salzburg.

Suggest doing the Romantic Road, there are many interesting small towns, most are medieval structures like Rothenberg.

Italy - Venice, Dolomites, Verona, Padua, Italian Lakes.
Austria - Salzburg, Hallstatt, Innsbruck, Tirol mountains.
Germany - Munich, Bavarian Alps, Neuschwanstein, Berchtesgaden/Zugspitze, Passau-Regensberg, Nuremberg.

Dolomites - the Val di Siusi
Munich, Salzburg, Venice, Lake Garda
Northern Italy is wonderful, so many fascinating towns like Trento, Brescia, Vicenza, but I'm not sure these are must-see unless you have a curiosity for history, art, etc...
Actually now that I think of it, I'm not sure what you mean by "must see".

I have to agree with Sam. I spent a night and part of two days in Zürich and don't regret it, but I would hardly call it a must see.

I've spent a couple months of my life in Bavaria, and I still feel I have much more to see.

This post sums up how I feel:
https://community.ricksteves.com/travel-forum/general-europe/must-see