This is all based on hard data we have collected from my company StartupDigest. direct link: www.thestartupdigest.com

The strongest 5 cities outside of the US for us (60% of our issues are outside of the US) based on subscribe size, growth, and click through rate are in this order:

1. London 2. Paris 3. Tel Aviv 4. Beijing 5. Toronto

Granted I haven't lived in any of these cities long enough to know whats it really like but these are the top cities from our data alone.

My advice would be to get in touch with with founders, hackers, and VC's in these areas to get the real feel for what these cities are like. Then visit your top choices, and cowork there for 2-4 weeks.

Hope this helps!