Chicago has one of the more interesting mix of old and new I have ever seen. I haven’t seen it in 30 years but I would imagine Frankfurt’s is pretty awesome these days.
Depending on the angle, San Francisco has a tremendous mixture, since the high rises are all in one pocket downtown, and the hills standout with much older buildings and streets.
I love seeing Santa Barbara from the water, because there are height limits of four stories, and strict architectural controls, with white stucco and Spanish tile roofs. It fits right into place on the landscape.
New York is an obvious member of this list. There are several buildings that were, at the time they were finished, the tallest building in the world. There are lots of old brownstones. There are the cast iron clad manufacturing buildings of the early 20th century. There are some examples of world-class architectural beauty and some examples of world-class dreck.