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.
Cost is low and it is a real adventure :-)
If anybody has a convincing idea, I'll come along (fluent german and english). Hungarian will work after a while.
Awesome mountain range. Great food. Low costs. Very friendly people. And two major hubs nearby with a growing startup culture (Cluj and Bucharest).
Barcelona seems to be starting to get into the startup scene.