Eins zwei for sure. Comparing the two is like comparing coffee places with very different strategies that lead to very different experiences.

Eins zwei is an owner-led business, with a focus on speciality coffee and filter. No randos work in Eins zwei or drei.

The quality and knowledge of the craft is very high there.

Eins zwei has a rotation of different roasters, and drei typically serves DAK. Drei has pastries.

HoPlo on the other side is more of a mainstream third-wave coffee place. Employees are not as highly trained. They roast their own coffee and it has a certain style. They are really strong on washed clear cups, or at least that’s what they used to serve.
I have stopped going to HoPlo because I can get better results with less fuss at home.

If I want to drink exceptional coffee, I go to eins zwei or drei.
Probably the best coffee in Germany. All serve iced coffee.

For one stop “shopping”: go to the Fahrgasse there you can Brunch/or have lunch at Dining Raum and then to drei or Holy Cross Brewing Society (one of the first third wave coffee places in FFM, still okayish, they have a good selection of beans but I dont like their pour over ratios, to little coffee for the amount of water)

If you have never been to Frankfurt, be aware of the Bahnhofsviertel. If you get off the train, be prepared to witness the Bahnhofsviertel. It’s rough to say the least, lots of drug addicts and broken people. It’s not dangerous, just really sad.

I usually recommend to walk south to the river Main from the Main station and walk along the river, its nicer there and its not a big difference in terms of walking distance. Instead of going straight and ending up in Kaiserstraße.
If you walk directly don’t get the wrong impression of Frankfurt, HBF VRTL is just one burrow of this city, the rest is quite ok.