Loved Berlin and Munich, though it's bee more than 20 years ago. Highly recommend both.

Have also been to Hamburg and Rostock. Frankfurt does not have a great reputation, but I defer to others.

From your list, I've only been to Munich. I was just there earlier this month. Really liked it as it was a great spot for day trips to elsewhere in the region. I'd love to come back in the summer.

I've heard Essen is gorgeous!

Berlin is phenomenal, there is a lot to do and see. It's very different from Munich, which is also a nice city. What time of year are you planning on going?

There is a lot more to Berlin than just the Berlin Wall. It has a huge number of museums. The Reichstag is worth a tour. Loads of monuments. Palace in Berlin and a massive (think Versaille) complex in nearby Potsdam.
Pfaueninsel (Peacock Island) is cool and fun https://en.wikipedia.org/wiki/Pfaueninsel
Jagdschloss Grunewald (https://en.wikipedia.org/wiki/Jagdschloss_Grunewald)

The Olympic Stadium, great to go swimming there if you go in summer. Berlin is a very green city, which most people don't necessarily see or realize. Lots of lakes, boating, canal/river. Forest.

Full disclosure, I'm from Berlin so maybe a tad biased.

Love Berlin and Munich. Frankfurt is meh. If you're looking for a city in the western part of the country, I'd do Cologne instead of Frankfurt.

There’s no reason to go to Frankfurt.

Agree with others that Berlin is fantastic! So much history. Really enjoyed the Stasi Museum. Munich is also fun with beautiful architecture. Hamburg is neat for a day trip. Do not miss the Miniatur Wunderland!

Frankfurt is a small finance oriented city. Not much to see there.

Recently went to Berlin. So much history and incredible sites. Potsdam is nearby and has a lot to see as well. Great shopping in Berlin too.

Like smaller cities/towns

Rothenburg
Oldenburg
Gottingen
Wismar
Wernigerode
Cochem
Lindau
Trier

Munich is incredible! Frankfurt- just get in and out of the airport honestly.

I’ve been to all three. One trip we flew into Berlin and stayed for a few days. The historical significance is on display at museums. We did the open bus tour, check point Charlie, Jewish ghetto, and shopping. Then took the train to Heidelberg. Flew home out of Frankfurt.

I’ve been to Munich. I like Nuremberg better. Munich felt like just another large city.

Frankfurt is ok. I prefer Heidelberg. You can see many castle, boat tour on the Rhein, and see castles. Wurzburg and the romantic road is great. It feels more authentically German.

Munich is lovely—we were there in September (during Oktoberfest) and enjoyed wandering around the city, the English Garden (big municipal park), and visiting the fest with our young kids. We also did a day trip to see the countryside and that Disney castle, which was really beautiful, though sort of a drag to ride the bus.

There’s a ton of great city stuff to do in Berlin—restaurants, museums, clubs, bars, shopping, etc.

I have only been to the airport in Frankfurt and don’t remember much.

Loved Berlin. Also we did the fairytale route drive, lots of small towns, beautiful nature, it was amaxing.

Berlin, Munich and Frankfurt are three that I would like to see. Has anyone been to these or others? Feedback?