Not sure of anyone went to more than one event -- aside from organizers obviously -- hence question can be directed to them. But event was the biggest success in terms of audience, topics, presenters?
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Hard to answer, because who other than the organizers (or really just Joel as I only went to 4 myself) has been to every single DevDays?
That said, I'd go with London, just based on size. It had 900+ attendees, which is almost 2x as large as the next one, which I believe was Toronto with 500. You need a fairly large group to get the network effects going full swing.