Berlin has an amazing startup scene with fairly open people. I got easily in contact with CEOs of Soundcloud and Blinkist. Try also to connect with hub:raum , which is the Telekom incubator in Berlin. Great stuff.

Other than that, I would probably go for Paris or Zurich, dependent on your startup idea.

In general: Try to find a place, where like-minded startups are situated. This swarming of common interests is indeed very valuable for skill, experience, and knowledge transfer.