The Sony-Center at Potsdamer Platz has long been my hotspot of choice because I can order wine at Josty's and sit outside and connect. But over the years the connection has been either weakened or overloaded. I try now rather to pick my hotels with free WiFi -which rules out most upscale hotels as, strangely enough, they charge.
German law makes really open hotspots difficult because the provider is legally responsible for the actions of all users. I'm pretty sure Deutsche Telekom is behind that.
I recently stayed at the very modest Hotel-Pension Arche
Kantstr. 104a D-10627 Berlin. It's WPA with a very simple code. The WiFi is strongest in the small breakfast room. The location is very near the S and U-Bahn Charlottenberg.