Probably Berlin. Vilnius or Tallinn could be interesting choices because of friendly business climate, lower taxes, competent and cheap talent, not far away, good tech culture and basically 0% chance that someone would try to outlaw your business (Germany probably has a bit higher chance but I guess it's also not that high). I think US do have some restrictions about exporting encryption to some (all?) foreign countries so you better look closer into that before making a decision. P.S. Stockholm also could be a good place. It's just quite expensive.