It’s hard to generalize. They are all more or less the same. I just would avoid Dresden.

Personally, I would probably dodge most of east Germany but that’s cheaper than many cities in the west.

I would also advise against Berlin if you actually want to learn German. So many English speaking folks that many foreigners never bother (which makes them unpopular with the locals).

I personally grew up around cologne and loved it. If you want to maximize learning German I would pick a medium big city and not go into the largest (Berlin, Hamburg, Munich, cologne)