Actually, don't miss Cove if you can help it.
Edit: Ansons is also good, but not unique to Frankfurt. You've probably got one in Munich already.
For this list, I'll say:
"Give me your tired, your poor,
Your huddled masses yearning to breathe free,
The wretched refuse of your teeming shore.
Send these, the homeless, tempest-tost to me,
I lift my lamp beside the golden door!"
Some help to populate the London section. I am not sure if any shop on Jermyn can be considered as a pure gentlemen's store on its own (New and Lingwood certainly is close), but the whole street altogether is one big shop.
London, United Kingdom